Refrigeration and Air Conditioning 3 E Part 13 ppsx

If the expansion valve is too tightly rated to accept lower condenser pressure, changethe expansion valve, possibly for the electronic type.. Compressors for dry expansion systemsmay be

1 Incorrect setting of head pressure controls

2 Dirty or choked spray nozzles in water tower or evaporativecondenser, so that the surface is not fully wetted

3 Non-condensible gas in circuit

4 Bad location of condensers, so that air recirculates

5 Undersized condensers

6 Dirty fins on air-cooled condensers

7 Fans not working or broken

8 Water strainers blocked

9 Undersize pumps fitted

10 Air in water circuit

While all these factors affect the good running of ment compressors, the effect is far worse with centrifugal machines,which can approach stall condition and so give a much reducedcooling duty

positive-displace-34.7 Maintenance

The good running order of equipment depends on the standards

of maintenance This is a running cost to be assessed with all theothers If it is found to be faulty, the investigation must considerwhat this is costing in terms of plant inefficiency and the expenditure

to reach acceptable standards This might be in the replacement orextra training of staff, or in contracting the work out If the latter,the cost must include supervisory expenses

34.8 Remedial action on existing equipment

The faults described above are largely self-revealing and most ofthem can be corrected or improved without a great deal of expen-diture The presence of separate metering devices should give animmediate indication of the savings made

34.9 Improved controls and equipment on

existing plant

Deficiencies on the original plant might be corrected by comparativelyminor improvements, changes and additions Each should be assessedfor its individual contribution to energy economy and how it mayimprove the performance of other parts of the system

1 Optimum-start controls

2 Ambient-biased set point controls

3 Modifications to give improved air and water flows, wherethese were shown to be deficient, i.e increase fan speeds,change fans, change pumps, improve ductwork or piping toreduce pressure losses.

4 Improved defrost control, to defrost coils only when and for

as long as necessary

5 Improved cold store door-operating mechanisms (see Figure15.8)

6 Improved condenser pressure control If the expansion valve

is too tightly rated to accept lower condenser pressure, changethe expansion valve, possibly for the electronic type

7 Automatically switch off plant which is not in use (boiler insummer, tower in winter, lights at night, etc.)

8 Switch off some of the cold store fans and coolers at night andweekends

9 Fit an automatic load-shedding maximum demand limiter

10 Resite condensers for better air flow

More drastic items may be:

1 Replace worn, obsolete or undersize compressors, evaporators

or condensers

2 Add new compressors, evaporators or condensers if these can

be shown to be economical

34.10 Design of systems for energy economy

Previous chapters have outlined the methods of estimating loads,choosing methods to achieve the required conditions, and how toselect and balance plant for correct operation They have also men-tioned the factors which will give economy in running costs.The maximum use should be made of energy-saving methods,where these may be applicable Some of these are:

1 Use of all fresh air for air-conditioning, if required in cold weather

2 Provide mid-season heating from condenser heat or heat pump(reverse-cycle) operation

3 Run plant at night on low-cost electricity and make ice, to usefor chilled water when load comes on (ice-bank)

4 Switch plant off for periods when electricity is at a premiumtariff

5 Two-speed or electronically speed-controlled motors for lowercompressor, fan and pump speeds at low load

6 Arrange the coolers within a cold store so that they will giveadequate air circulation at night when half of them are switchedoff

Much attention has been given in recent years to the power consumed

in the refrigeration process and the development of more efficientcompressors A few points to consider are:

1 Avoid high compression ratios on piston compressors

2 Avoid single-stage compression for very low temperatures

3 Avoid machines which are working at the upper or lower limits

of their range

4 Always ask the running power required at load conditions.The resulting system design will not be the lowest in first cost

34.11 Commitment to energy savings

A positive energy policy needs to be a company decision, taken atboardroom level and backed by boardroom authority, since it cutsacross departmental boundaries and may conflict with the opinions

of senior staff Typical objections are:

1 The capital, operating, maintenance and fuel costs come fromfour separate budgets, possibly accounted for by four differentmanagers, so these budgets need to be adjusted Separate fuelmeters are needed to prove the savings, which might otherwise

be held in question

2 There may be some disruption to normal working while theschemes are being carried out This may affect departments notconcerned directly with the programme

3 Staff may need to be released for training schemes

4 The improvements may need changes in operating techniqueswhich are thought to be adequate already

Some of these, such as the tightening of discipline of fork-lift truckdrivers, may provoke open conflict, which must be foreseen andheaded off

It is important to be able to quantify the results of the energyprogramme and make these known to all concerned A conservationprogramme of this sort is an ongoing process and should keep allstaff concerned alert to the possibilities of further improvements

35 Catalogue selection

35.1 General

Manufacturers will publish rating and application data for theirproducts, based on standard test conditions and for the more usualrange of uses They cannot be expected to have accurate figures forevery possible combination of conditions for an individual purpose,although most will produce estimates if asked

The widespread use of packaged units of all sizes requiresinterpretation of catalogue data by applications engineers, salesengineers, and others, and by the end user

The first step is to be certain of the basis of the published dataand consider in what ways this will be affected by different conditions.Revised figures can then usually be determined For extensiveinterpretation work, simple mathematical models of performancecan be constructed [69]

35.2 Compressors

Refrigeration compressors which will probably be used on floodedevaporators (R.717 and the larger machines generally) will be ratedwith the suction at saturated conditions, since there will be little or

no superheat in practice Compressors for dry expansion systemsmay be rated at a stated amount of superheat, commonly 8 K.There will be a pressure drop and heat gain in the suction line,and these are frequently ignored if the pipe run is short In othercases, some allowance must be made Both these factors will increasethe specific volume

Example 35.1 An ammonia compressor is rated at 312 kW withsaturated suction at – 15°C It is installed with a very long suctionline, causing a pressure drop of 0.4 bar, and picks up 6 K superheatfrom its evaporator condition Estimate the capacity loss

Evaporator pressure at – 15°C = 2.36 bar abs.

Suction pressure, 2.36 – 0.4 = 1.96 bar abs

Rating suction temperature = – 15.0°C

Actual suction temperature, – 15 + 6 K = – 9.0°C

The absolute gas pressures must be used in this calculation (seeSection 1.4)

The volume pumped by the compressor will remain about thesame, but the density of the gas is reduced, and thence the massflow

Using the General Gas Laws:

Halocarbon systems are almost invariably controlled by mechanical

or electronic thermostatic expansion valves, requiring a superheatsignal to operate the control The superheating of the suction gasinto the compressor will cause it to expand, resulting in a lowermass flow for a given swept volume Reduction of the superheatsetting of the expansion valve will therefore result in better use ofthe compressor The limit will be reached when there is insufficientsignal to work the expansion valve

Example 35.2 An R.22 compressor is rated at 15.9 kW when porating at – 5°C, with 8 K superheat Estimate the gain in capacity

eva-if it can be run safely with half the superheat

Rating suction temperature, – 5 + 8 = 3°C

= 276.15 KWorking suction temperature, – 5 + 4 = – 1°C

= 272.15 K

Ratio of mass pumped =

m m

T T

2 1 1 2

= + 276.15

272.15 = 1.015This gives a gain in capacity of about 1.5%, or 0.24 kW

There will also be a gain in usage of the evaporator coil and acorresponding rise in the evaporator temperature, giving a furtherincrease in compressor capacity This would need to be evaluatedfrom the compressor curves, but might be a further 1%

Example 35.3 A hermetic compressor is rated at 18.2 kW for R.22when evaporating at 7°C, suction superheated to 35°C, condensing

at 54°C, and with 8 K subcooling of the liquid Assuming that theinlet gas picks up another 30 K as it passes over the compressormotor, estimate the change in capacity if the suction is superheated

to 12°C

(a) Change in mass flow:

Compressor inlet temperature, rating, 35 + 30 = 65°C

= 338.15 Kactual, 12 + 30 = 42°C

= 315.15 K

m m

T T

2 1 1 2

= = 338.15

315.15 = 1.073

(b) Change in enthalpy (kJ/kg):

Enthalpy of suction gas at 35°C = 329.8

Enthalpy of suction gas at 12°C =000.0 311.7

Corrected working capacity, 18.2 × 0.96 = 17.5 kW

Example 35.4 An air-cooled condensing unit is rated at 13.7 kW onR.22 when evaporating at 5°C and with ambient air at 43°C Estimatethe duty with ambient air at 52°C

Some assumptions must be made regarding the condenser coilperformance, and this may have a ∆T of 14 K between the entering

air and condensing refrigerant and subcooling the liquid 5 K, withsuction gas entering the compressor with 6 K superheat

Rating condensing temperature, 43 + 14 = 57 °C

Working condensing temperature, 52 + 14 = 66 °CEnthalpy of suction gas at (5 + 6) = 11°C = 312.1 312.1Enthalpy of liquid at (57 – 5) = 52°C = 165.3

133.6146.8 0.68× 0.75 = 11.3 kWThis is approximate, but probably within 0.2 kW

35.4 Evaporators

The rating of an evaporator will be proportional to the temperaturedifference between the refrigerant and the cooled medium Sincethe latter is changing in temperature as it passes over the coolersurface (see Section 1.8), an accurate calculation for a particularload is tedious and subject error

To simplify the matching of air-cooling evaporators to condensingunits, evaporator duties are commonly expressed in basic ratings(see Figure 35.1), in units of kilowatts per kelvin (formerly in Britishthermal units per hour per degree Fahrenheit) This rating factor

is multiplied by the ∆T between the entering air and the refrigerant.

Example 35.5 An air-cooling evaporator has a mass air flow of 8.4kg/s and a published ‘rating’ of 3.8 kW/K What will be its rated

duty at – 15°C coldroom temperature with refrigerant at – 21°C?What is the true ln MTD?

Entering air temperature = – 15°CRefrigerant temperature = – 21°C

duty against evaporator temperature, line CD in Figure 35.2 The coil rating is plotted as the line AB, with A at the required coldroom (or ‘air-on’) temperature, and the slope of the line AB corresponding

to the basic rating The intersection of this line with the condensing

unit curve CD gives the graphical solution of the system balance

point Similar constructions for higher condenser air conditions

(EF, GH) or different room temperatures (A1B1) will show balancepoints for these conditions

The graph also indicates the change in evaporating temperature

Air in –15 ° C

In MTD 4.5 K

Air out –17.73 ° C

and coil duty when the ambient is lower or higher than the designfigure This will show if there is any necessity to control the evaporatingtemperature in order to keep the correct plant operation (See alsoSections 9.8 and 9.11.)

35.5 Reduction of air flow

Frequently, coil data will be available for a design air flow, but thesystem resistance reduces this flow to a lower value There is adouble effect: the lowering of the ln MTD and the lower heat transferfrom the coil by convection

The outer surface coefficient is the greatest thermal resistivity(compared with conduction through the coil material and the insidecoefficient), and a rough estimate of the total sensible heat flowchange can be made on the basis of [5] and [6]:

h = constant × (V )0.8

Example 35.6 An air cooling coil extracts 45 kW sensible heatwith air entering at 24°C and leaving at 18°C, with the refrigerantevaporating at 11°C Estimate the cooling capacity at 95, 90 and85% mass air flow

Design mass air flow

25 ° C

35 ° C

30 ° C 

Air onto condenser

C

E G

A

A1– 40 – 35 – 30 – 25 – 20 – 15

Air flow (%)

Air temperature on coil (°C) 24 24 24 24

This first estimate for the evaporator coil performance must now

be corrected for the change in compressor duty if it is a directexpansion coil, or of water temperature change if using chilledwater Another method is to re-calculate the basic rating figures atthe new air flows and plot these against compressor curves.With all calculations involving convective heat transfer, it must

be remembered that the figures are predictions based on previoustest data, and not precise

35.6 Room air-conditioners

The catalogue-rated cooling capacity of a room air-conditioner, ifnot qualified, will be based on ASHRAE Standard 16-1983 Thisspecifies test conditions of air onto the evaporator at 80°F dry bulb,50% relative humidity (26.7°C, 49.1% saturation), and air onto thecondenser at 95°F dry bulb, 75°F wet bulb (35°C and 23.9°C) Theoriginal basis for this specification was the ambient conditionprevailing in the mass-market area of the USA

For these units, British Standard 2582: Part 1, 1982 gives threesets of alternative rating conditions, corresponding to the ASHRAEStandard, for tropical, arid and temperate ambients They are:

Room air temperature Outside air temperature

test methods Any catalogue ratings quoting this ISO must be qualifiedwith test conditions.

Performance of the average commercial room air-conditioner at

BS.2852, condition C, will be some 10–15% lower than at condition

A, since it will evaporate some 5 K lower This reduction factor

should be applied to any unqualified unit rating if used under UKambient conditions

A further complication arises with the application to temperateconditions of room air-conditioners which have been designedprimarily for tropical markets These units typically work with asensible/total heat ratio of 0.7 Plotting this process line on thepsychrometric chart (see Figure 35.3) shows that the ADP will beabout 9°C

Dry bulb temperature ( ° C)

Low fan speed

B A C

Typical A SHRAE

25

20

15 10 7 2

° C) (sling)

For a room condition to BS.2852.C., and at full air flow, the ADPwill be just above freezing point If the unit is fitted with a low fanspeed control, the ADP can fall below freezing and the coil frostover Such units need to be fitted with a defrosting control and anallowance made for the time that the compressor will not be running

35.7 Product quality

All equipment should comply with the relevant British and otherStandards regarding dimensions, methods of determining ratings,compliance with safety regulations, robustness and general quality

of manufacture [70] BS.5750, Quality Systems, concentrates onthe subject of product quality as it affects design, manufacture andinstallation In addition to Standards, there are various Codes ofPractice [71, 72]

Most catalogues give insufficient information for comparisons ofquality, and an objective assessment may be difficult For majoritems of equipment and in cases of doubt, it will be helpful to visit

an existing installation or the factory

Where the standard is for compliance with a safety requirement,

a certificate to this effect should be provided, and may be demanded

by insurers

35.8 Analytical catalogue selection

Since a large proportion of refrigeration and air-conditioningequipment will be bought on the basis of catalogue data, an analyticalapproach should be adopted to ensure correct selection The

principles to be applied are those of value analysis – to start with the

basic need and no preconceived method, to consider all the differentmethods of satisfying the need, and to evaluate each of theseobjectively before moving towards a choice

The details of such an approach will vary considerably, and thefollowing guidelines should be taken as an indication of the factors

to be considered, rather than as an exhaustive list:

1 What is the basic need?

To cool something, a dry product, in air: temperature?

To keep something cool, a solid product

an enclosed space conditions?