The testing of boilers and other pressure ves-sels for compliance with safety codes is not the primary function of the testing and balancing firm; rather it is to verify and adjust oper-
Trang 21.10 1999 ASHRAE Applications Handbook (SI)
Capital and interest
Table 6 summarizes the interest and principle payments for this
example Annual payments are the product of the initial system cost
C s,init and the capital recovery factor CRF(i m,5) Also, Equation (10) can be used to calculate total discounted interest deduction directly.
Next, apply the capital recovery factor CRF(i′,5) and tax rate T inc to the total of the discounted interest sum.
Depreciation
Use the straight line depreciation method to calculate depreciation:
Next, discount the depreciation.
Finally, the capital recovery factor and tax are applied.
U.S tax code recommends estimating the salvage value prior to depreciating Then depreciation is claimed as the difference between the initial and salvage value, which is the way depreciation is treated in this example The more common practice is to initially claim zero sal- vage value, and at the end of ownership of the item, treat any salvage value as a capital gain.
Principal Payment, Current $
Outstanding Principal, Current $ PWF(i d , k)
Discounted Interest, Discounted $
Discounted Payment, Discounted $
Income Taxes
@50%, $
Net Cash Flow, b
$
Present Worth of Net Cash Flow
Trang 3Owning and Operating Costs 1.11
Summary of terms
Cash Flow Analysis Method The cash flow analysis method
accounts for costs and revenues on a period-by-period (e.g.,
year-by-year) basis, both actual and discounted to present value This
method is especially useful for identifying periods when net cash
flow will be negative due to intermittent large expenses
Example 10 An eight-year study for a $120 000 investment with
depreci-ation spread equally over the assigned period The benefits or incomes
are variable The marginal tax rate is 50% The rate of return on the
investment is required Table 7 has columns showing year, cash outlays,
income, depreciation, net taxable income, taxes and net cash flow.
Solution: To evaluate the effect of interest and time, the net cash flow
must be multiplied by the single payment present worth factor An
arbi-trary interest rate of 10% has been selected and the PWFsgl is obtained
by using Equation (4) Its value is listed in Table 7, column 8 Present
worth of the net cash flow is obtained by multiplying columns 7 and 8.
Column 9 is then added to obtain the total cash flow If year 0 is
ignored, an investment value is obtained for a 10% required rate of
return.
The same procedure is used for 15% interest (column 10, but the
PWF is not shown) and for 20% interest (column 11).
Discussion The interest at which the summation of present worth
of net cash flow is zero gives the rate of return In this example, the
investment has a rate of return by interpolation of about 15.4% If this
rate offers an acceptable rate of return to the investor, the proposal
should be approved; otherwise, it should be rejected.
Another approach would be to obtain an investment value at a
given rate of return This is accomplished by adding the present worth
of the net cash flows, but not including the investment cost In the
example, under the 10% given rate of return, $147 700 is obtained as an
investment value This amount, when using money that costs 10%,
would be the acceptable value of the investment.
Computer Analysis
Many computer programs are available that incorporate the
eco-nomic analysis methods described above These range from simple
macros developed for popular spreadsheet applications to more
comprehensive, menu-driven computer programs Commonly used
examples of the latter include Building Life-Cycle Cost (BLCC),
Life Cycle Cost in Design (LCCID), and PC-ECONPACK
BLCC was developed by the National Institute of Standards
and Technology (NIST) for the U.S Department of Energy
(DOE) The program follows criteria established by the Federal
Energy Management Program (FEMP) and the Office of
Manage-ment and Budget (OMB) It is intended for the evaluation of
energy conservation investments in nonmilitary government
buildings; however, it is also appropriate for similar evaluations
of commercial facilities
LCCID is an economic analysis program tailored to the needs of
the U.S Department of Defense (DOD) Developed by the U.S
Army Corps of Engineers and the Construction Engineering
Research Laboratory (USA-CERL), LCCID uses economic criteria
established by FEMP and OMB
PC-Econpack, developed by the U.S Army Corps of Engineers
for use by the DOD, uses economic criteria established by the OMB
The program performs standardized life-cycle cost calculations
such as net present value, equivalent uniform annual cost, SIR, and
discounted payback period
Macros developed for common spreadsheet programs generallycontain preprogrammed functions for the various life-cycle cost cal-culations Although typically not as sophisticated as the menu-driven programs, the macros are easy to install and easy to learn
Reference Equations
Table 8 lists commonly used discount formulas as addressed by
NIST Refer to NIST Handbook 135 (Ruegg) and Table 2.3 in that
handbook for detailed discussions
SYMBOLS
c = cooling system adjustment factor
C = total annual building HVAC maintenance cost
C e = annual operating cost for energy
C s,assess= assessed system value
C s,init = initial system cost
C s,salv = system salvage value at end of study period
C y = uniform annualized mechanical system owning, operating, and maintenance costs
CRF = capital recovery factor
CRF(i,n) = capital recovery factor for interest rate i and analysis period n CRF(i′,n) = capital recovery factory for interest rate i′ for items other
than fuel and analysis period n CRF(i″,n) = capital recovery factor for fuel interest rate i″ and analysis
period n CRF(i m ,n) = capital recovery factor for loan or mortgage rate i m and anal-
ysis period n
d = distribution system adjustment factor
D = depreciation during period k
Capital and interest −$2294.57
Total annualized cost −$1544.16
Table 8 Commonly Used Discount Formulas
Single compound-amount (SCA) equation
Single present value (SPW) equation
Uniform sinking-fund (USF) equation
Uniform capital-recovery (UCR) equation
Uniform compound-account (UCA) equation
Uniform present-value (UPW) equation
Modified uniform present-value (UPW*) equation
d = interest or discount rate
e = price escalation rate per period
Source: NIST Handbook 135 (Ruegg).
a Note that the USF, UCR, UCA, and UPW equations yield undefined answers when
d = 0 The correct algebraic forms for this special case would be as follows: USF formula, A = F/N; UCR formula, A = P/N; UCA formula, F = A ·n The UPW* equation also yields an undefined answer when e = d In this case, P = A0·n.
b The terms by which the known values are multiplied in these equations are the formulas for the factors found in discount factor tables Using acronyms to represent
the factor formulas, the discounting equaitons can also be written as F = P ·SCA,
P = F· SPW, A = F ·USF, A = P ·UCR, F = UCA, P = A· UPW, and P = A0· UPW*.
Trang 41.12 1999 ASHRAE Applications Handbook (SI)
D k,SL = depreciation during period k due to straight line depreciation
method
D k,SD = depreciation during period k due to sum-of-digits
deprecia-tion method
F = future value of a sum of money
h = heating system adjustment factor
i = compound interest rate per period
i d = discount rate per period
i m = market mortgage rate
i′ = effective interest rate for all but fuel
i″ = effective interest rate for fuel
I = insurance cost per period
ITC = investment tax credit
j = inflation rate per period
j e = fuel inflation rate per period
k = end of period(s) during which replacement(s), repair(s),
depreciation, or interest are calculated
M = maintenance cost per period
n = number of periods under analysis
P = present value of a sum of money
P k = outstanding principle on loan at end of period k
PMT = future equal payments
PWF = present worth factor
PWF(i d ,k) = present worth factor for discount rate i d at end of period k
PWF(i′,k) = present worth factor for effective interest rate i′ at end of
period k
PWF(i,n) sgl = single payment present worth factor
PWF(i,n) ser = present worth factor for a series of future equal payments
R k = net replacement, repair, or disposal costs at end of period k
T inc = net income tax rate
T prop = property tax rate
T salv = tax rate applicable to salvage value of system
REFERENCES
Akalin, M.T 1978 Equipment life and maintenance cost survey ASHRAE
Transactions 84(2):94-106.
DOE International performances measurement and verification protocol.
Publication No DOE/EE-0157 U.S Department of Energy.
Dohrmann, D.R and T Alereza 1986 Analysis of survey data on HVAC
maintenance costs ASHRAE Transactions 92(2A):550-65.
Easton Consultants 1986 Survey of residential heat pump service life and
maintenance issues Available from American Gas Association,
Arling-ton, VA (Catalog No S-77126).
Grant, E., W Ireson, and R Leavenworth 1982 Principles of engineering economy John Wiley and Sons, New York.
Haberl, J 1993 Economic calculations for ASHRAE Handbook Energy
Systems Laboratory Report No ESL-TR-93/04-07 Texas A&M
Univer-sity, College Station, TX.
Kreider, J and F Kreith 1982 Solar heating and cooling Hemisphere
Publishing, Washington, D.C.
Kreith, F and J Kreider 1978 Principles of solar engineering Hemisphere
Publishing, Washington, D.C.
Lippiatt, B.L 1994 Energy prices and discount factors for life-cycle cost
analysis 1993 Annual Supplement to NIST Handbook 135 and NBS Special Publication 709 NISTIR 85-3273.7 National Institute of
Standards and Technology, Gaithersburg, MD.
Lovvorn, N.C and C.C Hiller 1985 A study of heat pump service life.
ASHRAE Transactions 91(2B):573-88.
NIST Annual Supplement to NIST Handbook 135 National Institute of
Standards and Technology, Gaithersburg, MD.
NIST and DOE Building life-cycle cost (BLCC) computer program able from National Institute of Standards and Technology, Office of Applied Economics, Gaithersburg, MD.
Avail-OMB 1972 Guidelines and discount rates for benefit-cost analysis of
fed-eral programs Circular A-94 Office of Management and Budget,
Wash-ington, D.C.
Riggs, J.L 1977 Engineering economics McGraw-Hill, New York.
Ruegg, R.T Life-cycle costing manual for the Federal Energy Management
Program NIST Handbook 135 National Institute of Standards and
pro-USACE PC-Econpack computer program U.S Army Corps of Engineers, Huntsville, AL.
BIBLIOGRAPHY
ASTM 1992 Standard terminology of building economics Standard E 833
Rev A-92 American Society for Testing and Materials, West hoken, PA.
Consho-Kurtz, M 1984 Handbook of engineering economics: A guide for neers, technicians, scientists, and managers McGraw-Hill, New York Quirin, D.G 1967 The capital expenditure decision Richard D Win, Inc.,
engi-Homewood, IL.
Trang 5CHAPTER 36
TESTING, ADJUSTING, AND BALANCING
Terminology 36.1
General Criteria 36.1
Air Volumetric Measurement Methods 36.2
Balancing Procedures for Air Distribution 36.3
Variable Volume Systems 36.4
Principles and Procedures for Balancing Hydronic Systems 36.6
Water-Side Balancing 36.8
Hydronic Balancing Methods 36.9
Fluid Flow Measurement 36.11 Steam Distribution 36.14 Cooling Towers 36.15 Temperature Control Verification 36.15 Field Survey for Energy Audit 36.16 Testing for Sound and Vibration 36.18 Testing for Sound 36.18 Testing for Vibration 36.20
HE system that controls the environment in a building is a
Tdynamic entity that changes with time and use, and it must be
rebalanced accordingly The designer must consider initial and
sup-plementary testing and balancing requirements for commissioning
Complete and accurate operating and maintenance instructions that
include intent of design and how to test, adjust, and balance the
building systems are essential Building operating personnel must
be well trained, or qualified operating service organizations must be
employed to ensure optimum comfort, proper process operations,
and economy of operation
This chapter does not suggest which groups or individuals
should perform the functions of a complete testing, adjusting, and
balancing procedure However, the procedure must produce
repeat-able results that meet the intent of the designer and the requirements
of the owner Overall, one source must be responsible for testing,
adjusting, and balancing all systems As part of this responsibility,
the testing organization should check all equipment under field
con-ditions to ensure compliance
Testing and balancing should be repeated as the systems are
ren-ovated and changed The testing of boilers and other pressure
ves-sels for compliance with safety codes is not the primary function of
the testing and balancing firm; rather it is to verify and adjust
oper-ating conditions in relation to design conditions for flow,
tempera-ture, pressure drop, noise, and vibration ASHRAE Standard 111
outlines detailed procedures not covered in this chapter
TERMINOLOGY
Testing, adjusting, and balancing is the process of checking and
adjusting all the environmental systems in a building to produce the
design objectives This process includes (1) balancing air and water
distribution systems, (2) adjusting the total system to provide design
quantities, (3) electrical measurement, (4) establishing quantitative
performance of all equipment, (5) verifying automatic controls, and
(6) sound and vibration measurement These procedures are
accom-plished by checking installations for conformity to design,
measur-ing and establishmeasur-ing the fluid quantities of the system as required to
meet design specifications, and recording and reporting the results
The following definitions are used in this chapter Refer to
ASH-RAE Terminology of Heating, Ventilation, Air Conditioning, and
Refrigeration (1991) for additional definitions.
Test Determine quantitative performance of equipment.
Balance Proportion flows within the distribution system
(sub-mains, branches, and terminals) according to specified design
quantities
Adjust Regulate the specified fluid flow rate and air patterns at
the terminal equipment (e.g., reduce fan speed, adjust a damper)
Procedure An approach to and execution of a sequence of work
operations to yield repeatable results
Report forms Test data sheets arranged in logical order for
sub-mission and review The data sheets should also form the permanentrecord to be used as the basis for any future testing, adjusting, andbalancing
Terminal A point where the controlled medium (fluid or
energy) enters or leaves the distribution system In air systems,these may be variable air or constant volume boxes, registers,grilles, diffusers, louvers, and hoods In water systems, these may beheat transfer coils, fan coil units, convectors, or finned-tube radia-tion or radiant panels
GENERAL CRITERIA
Effective and efficient testing, adjusting, and balancing require asystematic, thoroughly planned procedure implemented by experi-enced and qualified staff All activities, including organization, cal-ibration of instruments, and execution of the actual work, should bescheduled Air-side must be coordinated with water-side work Pre-paratory work includes planning and scheduling all procedures, col-lecting necessary data (including all change orders), reviewing data,studying the system to be worked on, preparing forms, and makingpreliminary field inspections
Leakage can significantly reduce performance; therefore ductsmust be designed, constructed, and installed to minimize and con-trol air leakage During construction, all duct systems should besealed and tested for air leakage; and water, steam, and pneumaticpiping should be tested for leakage
Design Considerations
Testing, adjusting, and balancing begin as design functions, withmost of the devices required for adjustments being integral parts ofthe design and installation To ensure that proper balance can beachieved, the engineer should show and specify a sufficient number
of dampers, valves, flow measuring locations, and flow balancingdevices; these must be properly located in required straight lengths
of pipe or duct for accurate measurement The testing proceduredepends on system characteristics and layout The interactionbetween individual terminals varies with pressures, flow require-ments, and control devices
The design engineer should specify balancing tolerances gested tolerances are ±10% for individual terminals and branches innoncritical applications and ±5% for main ducts For critical appli-cations where differential pressures must be maintained, the follow-ing tolerances are suggested:
Exhaust and return air 0 to +10%
The preparation of this chapter is assigned to TC 9.7, Testing and Balancing.
Trang 6Testing, Adjusting, and Balancing 36.5
Varying Fan Speed Electrically This method of control, which
varies the voltage or frequency to the fan motor, is usually the most
efficient Some versions of motor drives may cause electrical noise
and affect other devices
In controlling VAV fan systems, the location of the static pressure
sensors is critical and should be field verified to give the most
rep-resentative point of operation After the terminal boxes have been
proportioned, the static pressure control can be verified by observing
static pressure changes at the fan discharge and the static pressure
sensor as the load is simulated from maximum airflow to minimum
airflow (i.e., set all terminal boxes to balanced airflow conditions
and determine whether any changes in static pressure occur by
plac-ing one terminal box at a time to minimum airflow, until all terminals
are placed at the minimal airflow setting) Care should be taken to
verify that the maximum to minimum air volume changes are within
the fan curve performance (speed or total pressure)
Diversity
Diversity may be used on a VAV system, assuming that the total
airflow is lower by design and that all terminal boxes will never
fully open at the same time Care should be taken to avoid duct
leak-age All ductwork upstream of the terminal box should be
consid-ered as pressure ductwork, whether in a low- or
medium-pressure system
A procedure to test the total air on the system should be
estab-lished by setting terminal boxes to the zero or minimum position
nearest the fan During peak load conditions, care should be taken to
verify that an adequate pressure is available upstream of all terminal
boxes to achieve design airflow to the spaces
Outside Air Requirements
Maintaining the space under a slight positive or neutral pressure
to atmosphere is difficult with all variable volume systems In most
systems, the exhaust requirement for the space is constant; hence,
the outside air used to equal the exhaust air and meet the minimum
outside air requirements for the building codes must also remain
constant Due to the location of the outside air intake and the
changes in pressure, this does not usually happen The outside air
should enter the fan at a point of constant pressure (i.e., supply fan
volume can be controlled by proportional static pressure control,
which can control the volume of the return air fan) Makeup air fans
can also be used for outside air control
Return Air Fans
If return air fans are required in series with a supply fan, the type
of control and sizing of the fans is most important Serious over- and
underpressurization can occur, especially during the economizer
cycle
Types of VAV Systems
Single-Duct VAV This system incorporates a
pressure-depen-dent or -indepenpressure-depen-dent terminal and usually has reheat at some
pre-determined minimal setting on the terminal unit or separate heating
system
Bypass This system incorporates a pressure-dependent damper,
which, on demand for heating, closes the damper to the space and
opens to the return air plenum Bypass sometimes incorporates a
constant bypass airflow or a reduced amount of airflow bypassed to
the return plenum in relation to the amount supplied to the space No
economical value can be obtained by varying the fan speed with this
system A control problem can exist if any return air sensing is done
to control a warm-up or cool-down cycle
VAV Using Single-Duct VAV and Fan-Powered,
Pressure-Dependent Terminals This system has a primary source of air
from the fan to the terminal and a secondary powered fan source that
pulls air from the return air plenum before the additional heat
source This system places additional maintenance of terminal ters, motors, and capacitors on the building owner In certain fan-powered boxes, backdraft dampers are a source of duct leakagewhen the system calls for the damper to be fully closed Typicalapplications include geographic areas where the ratio of heatinghours to cooling hours is low
fil-Double-Duct VAV This type of terminal incorporates two
sin-gle-duct variable terminals It is controlled by velocity controllersthat operate in sequence so that both hot and cold ducts can beopened or closed Some controls have a downstream flow sensor inthe terminal unit to maintain either the heating or the cooling Theother flow sensor is in the inlet controlled by the thermostat As thisinlet damper closes, the downstream controller opens the otherdamper to maintain the set airflow Often, low pressure in the deckscontrolled by the thermostat causes unwanted mixing of air, whichresults in excess energy use or discomfort in the space On mostdirect digital controls (DDC) inlet control on both ducts is favored
in lieu of the downstream controller
Balancing the VAV System
The general procedure for balancing a VAV system is
1 Determine the required maximum air volume to be delivered
by the supply and return air fans Diversity of load usuallymeans that the volume will be somewhat less than the outlettotal
2 Obtain fan curves on these units, and request information onsurge characteristics from the fan manufacturer
3 If an inlet vortex damper control is to be used, obtain the fanmanufacturer’s data pertaining to the deaeration of the fanwhen used with the damper If speed control is used, find themaximum and minimum speed that can be used on the project
4 Obtain from the manufacturer the minimum and maximumoperating pressures for terminal or variable volume boxes to beused on the project
5 Construct a theoretical system curve, including an approximatesurge area The system curve starts at the minimum inlet staticpressure of the boxes, plus system loss at minimum flow, andterminates at the design maximum flow The operating rangeusing an inlet vane damper is between the surge line intersec-tion with the system curve and the maximum design flow.When variable speed control is used, the operating range isbetween (a) the minimum speed that can produce the necessaryminimum box static pressure at minimum flow still in the fan’sstable range and (b) the maximum speed necessary to obtainmaximum design flow
6 Position the terminal boxes to the proportion of maximum fanair volume to total installed terminal maximum volume
7 Set the fan to operate at approximate design speed (increaseabout 5% for a full open inlet vane damper)
8 Check a representative number of terminal boxes If a widevariation in static pressure is encountered, or if the airflow at anumber of boxes is below minimum at maximum flow, checkevery box
9 Run a total air traverse with a pitot tube
10 Increase the speed if static pressure and/or volume are low Ifthe volume is correct, but the static is high, reduce the speed Ifthe static is high or correct, but the volume is low, check forsystem effect at the fan If there is no system effect, go over allterminals and adjust them to the proper volume
11 Run steps (7) through (10) with the return or exhaust fan set atdesign flow as measured by a pitot-tube traverse and with thesystem set on minimum outdoor air
12 Proportion the outlets, and verify the design volume with theVAV box on the maximum flow setting Verify the minimumflow setting
Trang 7Testing, Adjusting, and Balancing 36.7
Heat Transfer at Reduced Flow Rate
The typical heating-only hydronic terminal gradually reduces its
heat output as flow is reduced (Figure 1) Decreasing water flow to
50% of design reduces the heat transfer to 90% of that at full design
flow The control valve must reduce the water flow to 10% to reduce
the heat output to 50% The reason for the relative insensitivity to
changing flow rates is that the governing coefficient for heat
trans-fer is the air-side coefficient A change in internal or water-side
coefficient with flow rate does not materially affect the overall heat
transfer coefficient This means that (1) heat transfer for
water-to-air terminals is established by the mean water-to-air-to-water temperature
difference, (2) the heat transfer is measurably changed, and (3) a
change in the mean water temperature requires a greater change in
the water flow rate
A secondary concern also applies to heating terminals Unlike
chilled water, hot water can be supplied at a wide range of
temper-atures So, in some cases, an inadequate terminal heating capacity
caused by insufficient flow can be overcome by raising the supply
water temperature Design below the temperature limit of 120°C
(ASME low-pressure boiler code) must be considered
The previous comments apply to heating terminals selectedfor a 10 K temperature drop (∆t) and with a supply water temper-
ature of about 93°C Figure 2 shows the flow variation when 90%terminal capacity is acceptable Note that heating tolerancedecreases with temperature and flow rates and that chilled waterterminals are much less tolerant of flow variation than hot waterterminals
Dual-temperature heating/cooling hydronic systems are times completed and started during the heating season Adequateheating ability in the terminals may suggest that the system is bal-anced Figure 2 shows that 40% of design flow through the termi-nal provides 90% of design heating with 60°C supply water and a
some-5 K temperature drop Increased supply water temperature lishes the same heat transfer at terminal flow rates of less than 40%design
estab-In some cases, dual-temperature water systems may ence a decreased flow during the cooling season because of thechiller pressure drop; this could cause a flow reduction of 25%.For example, during the cooling season, a terminal that originallyheated satisfactorily would only receive 30% of the design flowrate
experi-While the example of reduced flow rate at ∆t = 10 K only affects
the heat transfer by 10%, this reduced heat transfer rate may havethe following negative effects:
1 The object of the system is to deliver (or remove) heat whererequired When the flow is reduced from the design rate, the sys-tem must supply heating or cooling for a longer period to main-tain room temperature
2 As the load reaches design conditions, the reduced flow rate isunable to maintain room design conditions
Terminals with lower water temperature drops have a greater erance for unbalanced conditions However, larger water flows arenecessary, requiring larger pipes, pumps, and pumping cost Also,automatic valve control is more difficult
tol-System balance becomes more important in terminals with alarge temperature difference Less water flow is required, whichreduces the size of pipes, valves, and pumps, as well as pumpingcosts A more linear emission curve gives better system control
Heat Transfer at Excessive Flow
The flow rate should not be increased above design in an effort toincrease heat transfer Figure 3 shows that increasing the flow to200% of design only increases heat transfer by 6% while increasingthe resistance or pressure drop 4 times and the power by the cube ofthe original power (pump laws)
Generalized Chilled Water Terminal—
Heat Transfer Versus Flow
The heat transfer for a typical chilled water coil in an air duct sus water flow rate is shown in Figure 4 The curves shown arebased on ARI rating points: 7.2°C inlet water at a 5.6 K rise withentering air at 26.7°C dry bulb and 19.4°C wet bulb
ver-The basic curve applies to catalog ratings for lower dry-bulbtemperatures providing a consistent entering air moisture content(e.g., 23.9°C dry bulb, 18.3°C wet bulb) Changes in inlet watertemperature, temperature rise, air velocity, and dry- and wet-bulbtemperatures will cause terminal performance to deviate from thecurves Figure 4 is only a general representation of the total heattransfer change versus flow for a hydronic cooling coil and doesnot apply to all chilled water terminals Comparing Figure 4 withFigure 1 indicates the similarity of the nonlinear heat transfer andflow for both the heating and the cooling terminal
Table 1 shows that if the coil is selected for the load, and the flow
is reduced to 90% of the load, three flow variations can satisfy thereduced load at various sensible and latent combinations
Fig 1 Effects of Flow Variation on Heat Transfer
from a Hydronic Terminal
(Design ∆t = 10 K and supply temperature = 93°C)
Fig 2 Percent of Design Flow Versus Design for
Various Supply Water Temperatures
Trang 836.10 1999 ASHRAE Applications Handbook (SI)
the desired curve can be determined from the manufacturer’s
rat-ings since these are published as (t ew − t ea) A second point is
established by observing that the heat transfer from air to water is
zero when (t ew− t ea) is zero (consequently, ∆t w = 0) With these
two points, an approximate performance curve can be drawn (see
Figure 6) Then, for any other (t ew− t ea), this curve is used to
deter-mine the appropriate ∆t w
Example 1 From the following manufacturer certified data, determine the
2 Construct a performance curve as illustrated in Figure 6.
3 From test data:
4 From Figure 6 read ∆t w = 5.4 K, which is required to balance water flow
at 0.1 L/s The water temperature difference may also be calculated as
pro-portion of the rate value as follows:
This procedure is useful for balancing terminal devices such as
finned tube convectors, where flow measuring devices do not exist
and where airflow measurements cannot be made It may also beused for cooling coils for sensible transfer (dry coil)
Flow Balancing by Total Heat Transfer This procedure
deter-mines water flow by running an energy balance around the coil.From field measurements of airflow, wet- and dry-bulb tempera-tures both upstream and downstream of the coil, and the difference
∆t w between the entering and leaving water temperatures, waterflow can be determined by the following equations:
(5)(6)(7)
where
Q w= water flow rate, L/s
q = load, W
q cooling= cooling load, W
q heating= heating load, W
Solution: From Equations (5) and (6),
The desired water flow is achieved by successive manual ments and recalculations Note that these temperatures can begreatly influenced by the heat of compression, stratification,bypassing, and duct leakage
adjust-General Balance Procedures
All the variations of balancing hydronic systems cannot belisted; however, the general method should balance the systemwhile minimizing operating cost Excess pump pressure (excessoperating power) can be eliminated by trimming the pump impeller.Allowing excess pressure to be absorbed by throttle valves adds alifelong operating cost penalty to the operation
The following is a general procedure based on setting the balancevalves on the site:
1 Develop a flow diagram if one is not included in the designdrawings Illustrate all balance instrumentation, and includeany additional instrument requirements
2 Compare pumps, primary heat exchangers, and specified minal units; and determine whether a design diversity factorcan be achieved
ter-3 Examine the control diagram and determine the control ments needed to obtain design flow conditions
adjust-∆t w 3
4.18 × 0.1 × 1 - 7.18 K
Test data
t ewb = entering wet-bulb temperature = 20.3°C
t lwb = leaving wet-bulb temperature = 11.9°C
Q a = airflow rate = 10 000 L/s
t lw = leaving water temperature = 15.0°C
t ew = entering water temperature = 8.6°C
From psychrometric chart
Trang 936.12 1999 ASHRAE Applications Handbook (SI)
For example, a manufacturer may test a boiler control valve with
40°C water Differential pressures from another test made in the
field at 120°C may be correlated with the manufacturer’s data by
using Equation (8) to account for the density differences of the two
tests
When differential heads are used to estimate flow, a density
cor-rection must be made because of the shape of the pump curve For
example, in Figure 8 the uncorrected differential reading for pumped
water with a density of 900 kg/m3 is 25 m; the gage conversion was
assumed to be for water with a density of 999 kg/m3 The
uncor-rected or false reading gives a 40% error in flow estimation
Differential Head Readout with Manometers
Manometers are used for differential pressure readout, especially
when very low differentials, great precision, or both, are required
But manometers must be handled with care; they should not be used
for field testing because fluid could blow out into the water and
rap-idly deteriorate the components A proposed manometer
arrange-ment is shown in Figure 9
Figure 9 and the following instructions provide accurate
manom-eter readings with minimum risk of blowout
1 Make sure that both legs of the manometer are filled with water
2 Open the purge bypass valve
3 Open valved connections to high and low pressure
4 Open the bypass vent valve slowly and purge air here
5 Open manometer block vents and purge air at each point
6 Close the needle valves The columns should zero in if the
manometer is free of air If not, vent again
7 Open the needle valves and begin throttling the purge bypass
valve slowly, watching the fluid columns If the manometer has an
adequate available fluid column, the valve can be closed and the
differential reading taken However, if the fluid column reaches
the top of the manometer before the valve is completely closed,
insufficient manometer height is indicated and further throttling
will blow fluid into the blowout collector A longer manometer or
the single gage readout method should then be used
An error is often introduced when converting millimetres of
gage fluid to the pressure difference (in kilopascals) of the test
fluid The conversion factor changes with test fluid temperature,
density, or both Conversion factors shown in Table 2 are to a water
base, and the counterbalancing water height H (Figure 9) is at room
temperature
Orifice Plates, Venturi, and Flow Indicators
Manufacturers provide flow information for several devices used
in hydronic system balance In general, the devices can be classified
as (1) orifice flowmeters, (2) venturi flowmeters, (3) velocityimpact meters, (4) pitot-tube flowmeters, (5) bypass spring impactflowmeters, (6) calibrated balance valves, (7) turbine flowmeters,and (8) ultrasonic flowmeters
The orifice flowmeter is widely used and is extremely accurate.
The meter is calibrated and shows differential pressure versus flow.Accuracy generally increases as the pressure differential across themeter increases The differential pressure readout instrument may
be a manometer, differential gage, or single gage (Figure 7)
The venturi flowmeter has lower pressure loss than the orifice
plate meter because a carefully formed flow path increases velocityhead recovery The venturi flowmeter is placed in a main flow linewhere it can be read continuously
Velocity impact meters have precise construction and
calibra-tion The meters are generally made of specially contoured glass orplastic, which permits observation of a flow float As flowincreases, the flow float rises in the calibrated tube to indicate flowrate Velocity impact meters generally have high accuracy
A special version of the velocity impact meter is applied tohydronic systems This version operates on the velocity pressuredifference between the pipe side wall and the pipe center, whichcauses fluid to flow through a small flowmeter Accuracy depends
on the location of the impact tube and on a velocity profile that responds to theory and the laboratory test calibration base Gener-
cor-ally, the accuracy of this bypass flow impact or differential velocity
pressure flowmeter is less than a flow-through meter, which canoperate without creating a pressure loss in the hydronic system
The pitot-tube flowmeter is also used for pipe flow
measure-ment Manometers are generally used to measure velocity pressuredifferences because these differences are low
The bypass spring impact flowmeter uses a defined piping
pressure drop to cause a correlated bypass side branch flow Theside branch flow pushes against a spring that increases in lengthwith increased side branch flow Each individual flowmeter is cali-brated to relate extended spring length position to main flow Thebypass spring impact flowmeter has, as its principal merit, a directreadout However, dirt on the spring reduces accuracy The bypass
Table 2 Differential Pressure Conversion to Head
Fluid Density,
kg/m 3
Approximate Corresponding Water Temperature, °C
Metre Fluid Head Equal to 1 kPa a
a Differential kPa readout is multiplied by this number to obtain metres fluid head
when gage is calibrated in kPa.
Fig 9 Fluid Manometer Arrangement for Accurate Reading and Blowout Protection
Trang 10Testing, Adjusting, and Balancing 36.13
is opened only when a reading is made Flow readings can be taken
at any time
The calibrated balance valve is an adjustable orifice flowmeter.
Balance valves can be calibrated so that a flow/pressure drop
rela-tionship can be obtained for each incremental setting of the valve A
ball, rotating plug, or butterfly valve may have its setting expressed
in percent open or degree open; a globe valve, in percent open or
number of turns The calibrated balance valve must be
manufac-tured with precision and care to ensure that each valve of a particular
size has the same calibration characteristics
The turbine flowmeter is a mechanical device The velocity of
the liquid spins a wheel in the meter, which generates a 4 to 20 mA
output that may be calibrated in units of flow The meter must be
well maintained, as wear or water impurities on the bearing may
slow the wheel, and debris may clog or break the wheel
The ultrasonic flowmeter senses sound signals, which are
cali-brated in units of flow The ultrasonic metering station may be
installed as part of the piping, or it may be a strap-on meter In either
case, the meter has no moving parts to maintain, nor does it intrude
into the pipe and cause a pressure drop Two distinct types of
ultra-sonic meter are available: (1) the transit time meter for HVAC or
clear water systems and (2) the Doppler meter for systems handling
sewage or large amounts of particulate matter
If any of the above meters are to be useful, the minimum distance
of straight pipe upstream and downstream, as recommended by the
meter manufacturer and flow measurement handbooks, must be
adhered to Figure 10 presents minimum installation suggestions
Using a Pump as an Indicator
Although the pump is not a meter, it can be used as an indicator
of flow together with the other system components Differential
pressure readings across a pump can be correlated with the pump
curve to establish the pump flow rate Accuracy depends on (1)
accuracy of readout, (2) pump curve shape, (3) actual conformance
of the pump to its published curve, (4) pump operation without
cav-itation, (5) air-free operation, and (6) velocity pressure correction
When a differential pressure reading must be taken, a single gage
with manifold provides the greatest accuracy (Figure 11) The pump
suction to discharge differential can be used to establish pump
dif-ferential pressure and, consequently, pump flow rate The single
gage and manifold may also be used to check for strainer clogging
by measuring the pressure differential across the strainer
If the pump curve is based on fluid head, pressure differential, as
obtained from the gage reading, needs to be converted to head,
which is pressure divided by the fluid density and gravity The pump
differential head is then used to determine pump flow rate (Figure
12) As long as the differential head used to enter the pump curve is
expressed as head of the fluid being pumped, the pump curve shown
by the manufacturer should be used as described The pump curve
may state that it was defined by test with 30°C water This is
unim-portant, since the same curve applies from 15 to 120°C water, or to
any fluid within a broad viscosity range
Generally, pump-derived flow information, as established by the
performance curve, is questionable unless the following precautions
are observed:
1 The installed pump should be factory calibrated by a test toestablish the actual flow-pressure relationship for that particularpump Production pumps can vary from the cataloged curvebecause of minor changes in impeller diameter, interior castingtolerances, and machine fits
2 When a calibration curve is not available for a centrifugal pumpbeing tested, the discharge valve can be closed briefly to estab-lish the no-flow shutoff pressure, which can be compared to thepublished curve If the shutoff pressure differs from that pub-lished, draw a new curve parallel to the published curve Whilenot exact, the new curve will usually fit the actual pumpingcircumstance more accurately Clearance between the impellerand casing minimize the danger of damage to the pump during ano-flow test, but manufacturer verification is necessary
3 Differential pressure should be determined as accurately as sible, especially for pumps with flat flow curves
pos-4 The pump should be operating air-free and without cavitation
A cavitating pump will not operate to its curve, and differentialreadings will provide false results
5 Ensure that the pump is operating above the minimum net tive suction pressure
posi-6 Power readings can be used (1) as a check for the operatingpoint when the pump curve is flat or (2) as a reference checkwhen there is suspicion that the pump is cavitating or providingfalse readings because of air
Fig 10 Minimum Installation Dimensions for Flowmeter
Fig 11 Single Gage for Differential Readout Across
Pump and Strainer
Fig 12 Differential Pressure Used to Determine Pump Flow
Trang 11Testing, Adjusting, and Balancing 36.17
Data sheets needed for energy conservation field surveys
con-tain different and, in some cases, more comprehensive information
than those used for testing, adjusting, and balancing Generally,
the energy engineer determines the degree of fieldwork to be
per-formed; data sheets should be compatible with the instructions
received
Building Systems
The most effective way to reduce building energy waste is to
identify, define, and tabulate the energy load by building system
For this purpose, load is defined as the quantity of energy used in a
building, or by one of its subsystems, for a given period By
follow-ing this procedure, the most effective energy conservation
opportu-nities can be achieved more quickly because high priorities can be
assigned to systems that consume the most energy
A building can be divided into nonenergized systems and
ener-gized systems Nonenerener-gized systems do not require outside energy
sources such as electricity and fuel Energized systems (e.g.,
mechanical and electrical systems) require outside energy
Ener-gized and nonenerEner-gized systems can be divided into subsystems
defined by function Nonenergized subsystems are (1) building site,
envelope, and interior; (2) building use; and (3) building operation
Building Site, Envelope, and Interior The site, envelope, and
interior should be surveyed to determine how they can be modified
to reduce the building load that the mechanical and electrical
sys-tems must meet without adversely affecting the building’s
appear-ance It is important to compare actual conditions with conditions
assumed by the designer, so that the mechanical and electrical
sys-tems can be adjusted to balance their capacities to satisfy actual
needs
Building Use These loads can be classified as people occupancy
loads or people operation loads People occupancy loads are related
to schedule, density, and mixing of occupancy types (e.g., process
and office) People operation loads are varied, and include (1)
oper-ation of manual window shading devices; (2) setting of room
ther-mostats; and (3) conservation-related habits such as turning off
lights, closing doors and windows, turning off energized equipment
when not in use, and not wasting domestic hot or chilled water
Building Operation This subsystem consists of the operation
and maintenance of all the building subsystems The load on the
building operation subsystem is affected by factors such as (1) the
time at which janitorial services are performed, (2) janitorial crew
size and time required to clean, (3) amount of lighting used to
perform janitorial functions, (4) quality of the equipment
mainte-nance program, (5) system operational practices, and (6)
equip-ment efficiencies
Building Energized Systems
The energized subsystems of the building are generally
plumb-ing, heatplumb-ing, ventilatplumb-ing, coolplumb-ing, space conditionplumb-ing, control,
elec-trical, and food service Although these systems are interrelated and
often use common components, logical organization of data
requires evaluating the energy use of each subsystem as
indepen-dently as possible In this way, proper energy conservation
mea-sures for each subsystem can be developed
Process Loads
In addition to building subsystem loads, the process load in most
buildings must be evaluated by the energy field auditor Most tasks
not only require energy for performance, but also affect the energy
consumption of other building subsystems For example, if a
pro-cess releases large amounts of heat to the space, the propro-cess
con-sumes energy and also imposes a large load on the cooling system
Guidelines for Developing a Field Study Form
A brief checklist follows that outlines requirements for a fieldstudy form needed to conduct an energy audit
Inspection and Observation of All Systems Record physical
and mechanical condition of the following:
• Fan blades, fan scroll, drives, belt tightness, and alignment
• Filters, coils, and housing tightness
• Ductwork (equipment room and space, where possible)
• Strainers
• Insulation ducts and piping
• Makeup water treatment and cooling tower
Interview of Physical Plant Supervisor Record answers to the
following survey questions:
• Is the system operating as designed? If not, what changes havebeen made to ensure its performance?
• Have there been modifications or additions to the system?
• If the system has been a problem, list problems by frequency ofoccurrence
• Are any systems cycled? If so, which systems and when, andwould building load permit cycling systems?
Recording System Information Record the following system/
grav-• Air-handling equipment—fans (supply, return, and exhaust):manufacturer, model, size, type, and class; dampers (vortex,scroll, or discharge); motors: manufacturer, power requirement,full load amperes, voltage, phase, and service factor
• Chilled and hot water coils—area, tubes on face, fin spacing, andnumber of rows (coil data necessary when shop drawings are notavailable)
• Terminals—high-pressure mixing box: manufacturer, model, andtype (reheat, constant volume, variable volume, induction);grilles, registers, and diffusers: manufacturer, model, style, andloss coefficient to convert field-measured velocity to flow rate
• Main heating and cooling pumps, over 3.5 kW—manufacturer,pump service and identification, model, size, impeller diameter,speed, flow rate, head at full flow, and head at no flow; motordata: power, speed, voltage, amperes, and service factor
• Refrigeration equipment—chiller manufacturer, type, model,serial number, nominal tons, input power, total heat rejection,motor (kilowatts, amperes, volts), chiller pressure drop, enteringand leaving chilled water temperatures, condenser pressure drop,condenser entering and leaving water temperatures, runningamperes and volts, no-load running amperes and volts
• Cooling tower—manufacturer, size, type, nominal cooling ity, range, flow rate, and entering wet-bulb temperature
capac-• Heating equipment—boiler (small through medium) turer, fuel, energy input (rated), and heat output (rated)
manufac-Recording Test Data Record the following test data:
• Systems in normal mode of operation (if possible)—fan motor:running amperes and volts and power factor (over 3.5 kW); fan:speed, total air (pitot-tube traverse where possible), and staticpressure (discharge static minus inlet total); static profile drawing(static pressure across filters, heating coil, cooling coil, anddampers); static pressure at ends of runs of the system (identify-ing locations)
• Cooling coils—entering and leaving dry- and wet-bulb tures, entering and leaving water temperatures, coil pressure drop(where pressure taps permit and manufacturer’s ratings can be
Trang 12tempera-Testing, Adjusting, and Balancing 36.19
Background sound measurements generally have to be made (1)
when the specification requires that the sound levels from HVAC
equipment only, as opposed to the sound level in a space, not exceed
a certain specified level; (2) when the sound level in the space
exceeds a desirable level, in which case the noise contributed by the
HVAC system must be determined; and (3) in residential locations
where little significant background noise is generated during the
evening hours and where generally low allowable noise levels are
specified or desired Because background noise from outside
sources such as vehicular traffic can fluctuate widely, sound
mea-surements for residential locations are best made in the normally
quiet evening hours
Sound Testing
Ideally, a building should be completed and ready for occupancy
before sound level tests are taken All spaces in which readings will
be taken should be furnished with drapes, carpeting, and furniture,
as these affect the room absorption and the subjective quality of the
sound In actual practice, since most tests have to be conducted
before the space is completely finished and furnished for final
occu-pancy, the testing engineer must make some allowances Because
furnishings increase the absorption coefficient and reduce to 4 dB
the sound pressure level that can be expected between most live and
dead spaces, the following guidelines should suffice for
measure-ments made in unfurnished spaces If the sound pressure level is
5 dB or more over specified or desired criterion, it can be assumed
that the criterion will not be met, even with the increased absorption
provided by furnishings If the sound pressure level is 0 to 4 dB
greater than specified or desired criterion, recheck when the room is
furnished to determine compliance
Follow this general procedure:
1 Obtain a complete set of accurate, as-built drawings and
specifi-cations, including duct and piping details Review specifications
to determine sound and vibration criteria and any special
instruc-tions for testing
2 Visually check for noncompliance with plans and specifications,
obvious errors, and poor workmanship Turn system on for aural
check Listen for noise and vibration, especially duct leaks and
loose fittings
3 Adjust and balance equipment, as described in other sections, so
that final acoustical tests are made with the HVAC as it will be
operating It is desirable to perform acoustical tests for both
sum-mer and winter operation, but where this is not practical, make
tests for the summer operating mode, as it usually has the
poten-tial for higher sound levels Tests must be made for all
mechan-ical equipment and systems, including standby
4 Check calibration of instruments
5 Measure sound levels in all areas as required, combining
mea-surements as indicated in item 3 if equipment or systems must be
operated separately Before final measurements are made in any
particular area, survey the area using an A-weighted scale
read-ing (dBA) to determine the location of the highest sound
pres-sure level Indicate this location on a testing form, and use it for
test measurements Restrict the preliminary survey to determine
location of test measurements to areas that can be occupied by
standing or sitting personnel For example, measurements would
not be made directly in front of a diffuser located in the ceiling,
but they would be made as close to the diffuser as standing or
sit-ting personnel might be situated In the absence of specified
sound criteria, the testing engineer should measure sound
pres-sure levels in all occupied spaces to determine compliance with
criteria indicated in Chapter 46 and to locate any sources of
excessive or disturbing noise
6 Determine whether background noise measurements must be
made
(a) If specification requires determination of sound level fromHVAC equipment only, it will be necessary to take back-ground noise readings by turning HVAC equipment off.(b) If specification requires compliance with a specific noiselevel or criterion (e.g., sound levels in office areas shall notexceed 35 dBA), ambient noise measurements must be madeonly if the noise level in any area exceeds the specifiedvalue
(c) For residential locations and areas requiring very low noise,such as sound recording studios and locations that are usedduring the normally quieter evening hours, it is usuallydesirable to take sound measurements in the evening and/ortake ambient noise measurements
7 For outdoor noise measurements to determine noise radiated byoutdoor or roof-mounted equipment such as cooling towers andcondensing units, the section on Sound Control for OutdoorEquipment in Chapter 46, which presents proper procedure andnecessary calculations, should be consulted
Noise Transmission Problems
Regardless of the precautions taken by the specifying engineerand the installing contractors, situations can occur where the soundlevel exceeds specified or desired levels, and there will be occa-sional complaints of noise in completed installations A thoroughunderstanding of Chapter 46 and the section on Testing for Vibra-tion in this chapter is desirable before attempting to resolve anynoise and vibration transmission problems The following isintended as an overall guide rather than a detailed problem-solvingprocedure
All noise transmission problems can be evaluated in terms of thesource-path-receiver concept Objectionable transmission can beresolved by (1) reducing the noise at the source by replacing defec-tive equipment, repairing improper operation, proper balancing andadjusting, and replacing with quieter equipment; (2) attenuating thepaths of transmission with silencers, vibration isolators, and walltreatment to increase transmission loss; and (3) reducing or maskingobjectionable noise at the receiver by increasing room absorption orintroducing a nonobjectionable masking sound The following dis-cussion includes (1) ways to identify actual noise sources using sim-ple instruments or no instruments and (2) possible corrections.When troubleshooting in the field, the engineer should listen tothe offending sound The best instruments are no substitute for care-ful listening, as the human ear has the remarkable ability to identifycertain familiar sounds such as bearing squeak or duct leaks and isable to discern small changes in frequency or sound character thatmight not be apparent from meter readings only The ear is also agood direction and range finder; because noise generally gets louder
as one approaches the source, direction can often be determined byturning the head Hands can also identify noise sources Air jets fromduct leaks can often be felt, and the sound of rattling or vibrating pan-els or parts often changes or stops when these parts are touched
In trying to locate noise sources and transmission paths, theengineer should consider the location of the affected area In areasthat are remote from equipment rooms containing significant noiseproducers but adjacent to shafts, noise is usually the result of struc-ture-borne transmission through pipe and duct supports andanchors In areas adjoining, above, or below equipment rooms,noise is usually caused by openings (acoustical leaks) in the sepa-rating floor or wall or by improper, ineffective, or maladjustedvibration isolation systems
Unless the noise source or path of transmission is quite obvious,the best way to identify it is by eliminating all sources systemati-cally as follows:
1 Turn off all equipment to make sure that the objectionablenoise is caused by the HVAC If the noise stops, the HVACcomponents (compressors, fans, and pumps) must be operated