Refrigeration and Air Conditioning 3 E Part 12 pps

Ensure safe operation at all times The control system will consist of a loop, with detector sensor,controller and controlled device.. Sinceall devices have some time lag in operation, th

3 Provide maximum efficiency and economy of operation

4 Ensure safe operation at all times

The control system will consist of a loop, with detector (sensor),controller and controlled device The communication between theseparts of the loop will be electric, pneumatic or mechanical (seeFigure 31.1) [62]

Flow of

energy

Controlled device

Space or process

Detector (sensor)

Control systems 325

two-position will be set to actuate at upper and lower limits, and willrespond when its sensitive element reaches these set limits Sinceall devices have some time lag in operation, the controlled conditionwill overshoot to some extent, depending on the time lag of thedetector and the extent to which the rate of supply of energy to theprocess exceeds the load (See Figure 31.2.) The range of the controlwill therefore be the differential of the detector plus the upper andlower overshoots under load

Time (a)

Figure 31.2 Limits of controlled variable with two-position control.(a) Capacity closely matched to load (b) Capacity much greaterthan load

Two-position detectors such as thermostats can be fitted with ananticipatory bias to reduce the amount of overshoot In suchinstruments, a small bias heater accelerates the action of the control

An alternative method to reduce overshoot is to introduce a timingdevice so that it acts intermittently Where the two-position sensor

is also the controller it provides only two plant outputs, maximum

or zero

A two-position detector can be used to operate a floating control

At the upper limit it will operate the control in one direction and

if it reaches the lower limit it will operate the control in the other

direction Between the two limits the control is not actuated (Seealso Section 31.4.)

Two-position detectors can be classified according to the purpose:Thermostatic bimetal

liquid expansionsolid expansionvapour pressure

bellowsbourdon tube

a working instruction to the controlled device Proportional detectorsinclude:

Temperature those above, plus

electrical resistance of a metal or a semiconductorthermocouple

infrared radiation

piezo-electricFluid flow that above, plus

electronic, Doppler effectorifice plate and manometer

electronic timing devices

resistance of a hygroscopic salt

Control systems 327

31.3 Controllers

If a controller is used with an on–off detector, it functions only as

an amplifier to transmit the detector signal to the controlled device

It can modify the speed of this action by a bias or by a slow-speedoperating motor, as in the floating control

The floating control normally takes the form of a slowly rotatingreversible motor moving a valve or operating a sequence of camswhich control, in turn, steps of plant capacity As the detector reachesits upper or lower limit it energizes the motor to advance or reducethe valve opening or the steps of plant capacity When the conditionhas been satisfied and the detector moves away from the limit, themotor stops The motor may be a solid-state timing or pulsing device.Some proportional detectors are combined in the same instrumentwith a suitable transducer which can perform some of the functions

of a controller For example, for pneumatic systems the primarysensing element actuates a variable air jet, thus modulating an airpressure which is transmitted to a further controller or direct to thecontrolled device Electric and electronic detectors such as theinfrared detector include the sensing and amplifying circuits of theinstrument

Controllers generally for use with proportional detectors willmeasure the displacement of the signal from a pre-set value andtransmit a proportional signal to the controlling device They mayalso be able to measure the rate of change of that signal (derivative)

or be able to modify the rate of change of the output signal (integral).The effect of these capabilities is to anticipate the deviation and sogive better response to changes of load A controller havingproportional, derivative and integral actions is known as a three-term controller

A controller may be arranged to accept input signals from morethan one detector, e.g the flow temperature of a hot water heatingsystem may be raised at the request of an outdoor detector if theambient falls, or may start the heating earlier in the morning topre-heat the building before it is occupied; a servo back pressureregulation valve (Figure 9.5) can respond both to evaporator pressureand load temperature With the advent of microcomputer devicesalmost any combination of signals can be processed by an electroniccontroller, providing the output signals can be made coherent andnot conflicting

Pneumatic controllers, which may include part of the sensinginstrument, are supplied with compressed air at 1 bar gauge which

is allowed to escape from an orifice controlled by a detector Theresulting pressure modulates about 0.4 bar and is used in a servo

piston, diaphragm or bellows to actuate the controlled device (SeeFigure 31.3.)

Controlled device Variable

orifice Sensor

Air nozzle

Figure 31.3 Pneumatic operation of controlled device

31.4 Controlled devices

Controlled devices commonly consist of an actuator, which acceptsthe signal from the controller and works the final element Typicalexamples are as follows:

motormotorized valvedampers

2 Electric solenoid operating solenoid valve

3 Modulated electronic operating magnetically positioned

control

4 Pneumatic pressure operating pneumatic relay

damper positionerThe effect of a controlled device may not be proportional to itsmovement In particular, the shape of valve plugs and the angle ofopening of dampers will not give a linear result, and the signal fromthe controller must take this into account [10, 62]

31.5 Controls communications

Communications between the component parts of a controls systemmay be by:

Low voltage, electronic circuits

Low voltage, thermocouple (microvolts)

Mechanical devices need careful installation to ensure that there is

no distortion of the parts This is especially the case with dampermechanisms, which need maintenance and periodic inspection toensure they are working correctly

Where fluid pressure is carried by a capillary tube, such as withthe thermostatic expansion valve or pressure switches, the tubeshould be installed with due attention to the risk of it chafing againstmetal edges and wearing through Tubes to manometers are usually

in plastic, but may be copper These must be carefully tested forleaks, as they are transmitting very low pressures

Mains voltage communications must be run according to IEEWiring or the appropriate safety regulations In particular, thesemay cause interference with telephonic, computer and otherelectronic signals carried in or near the same conduit In the sameway, electronic control signals may suffer interference Thermocouplesignals are very low voltage d.c and should be run as far as possiblewith unbroken conductors Terminal boxes should be compact andinsulated from sudden temperature changes Terminals must betight

Pneumatic controls are used widely in hazardous situations such

as chemical plants and oil refineries The same risk of chafing applies

as with capillary tubes Pneumatic tubing is more usually in copperand is correctly secured

Optical fibres are not yet very much in use, but there is nointerference between them and electrical signals of any sort Forthis reason their use will probably become more widespread Line-of-sight optical signals require that no obstruction is inserted at anytime Such points are easily noticed when installing and com-missioning, but are not so obvious if a malfunction occurs at a laterdate

Remote plant is sometimes controlled or monitored by radiolink This is subject to interference and should only be considered

if the cost of a permanent or telephonic connection is uneconomic

31.6 Control system planning

Control systems can quickly escalate into unmanageable complexity,and the initial approach to the design of a suitable control systemshould examine the purpose of each item, and the effect on others,

to eliminate those which are not essential

The action of a control may combine two or more of the purposes,

as set out in Section 31.1, which may then be interdependent It ismore informative to consider the action of a control and examinewhat purpose it may serve in the circuit

Controls for economic operation should ensure that functionsare shut down when not needed (the boiler in summer and thechiller in winter) Optimum start controls now complement thestarting clock, to advance or retard the starting time according tothe ambient

In planning a control system, a flow diagram is needed to indicatewhat may influence each item of plant In many diagrams it will beseen that complexity arises and two items work in conflict A typicalinstance is the cooling and dehumidifying of air, to a room conditionlower than design, with concurrent operation of a humidifier.Since most controls will be electrical and largely of the two-positiontype, it is a convenient notation to set out the initial control scheme

as an electrical circuit and in ‘book page’ form, i.e from left toright and line by line, to indicate the sequence of operation, withthe controlled device always in the right-hand column This analysisshould indicate the different items which might act to produce afinal effect and bring errors to light Figure 31.4 is a simplifiedcontrol circuit for a small air-conditioning system Non-electricalitems can be shown on the same initial scheme, possibly with dottedlines to indicate a non-electrical part of the system The possibilities

of abnormal operation should be examined, and grouped as systemnot working, system unsafe and system dangerous, and protectedaccordingly The last category requires two independent safetycontrols or one control and an alarm

Complex timing and logic controlling, monitoring and indicationcan now be carried out with programmable computer-type devices,using algorithms stored in RAM or EPROM These save the formercomplex arrangements of sequencing and interlock relays and timersbut still require the same attention to planning and design of thecircuit In all cases, a copy of the basic control diagram should beleft with the device, to inform users and service staff of the plan ofthe control system, and any subsequent modifications updated onthis diagram

Hand-Pump selector

Pump selector Hand-

auto Flow switch

Low limit safety Multistep thermostat

High pressure Low pressure

Winter thermostat

Flow switch

High limit safety Hot waterthermostat

Crank case heater

Cold water pump

Hot water pump Hot water pump Cold water pump

Condenser fan Compressor

100% capacity 66% capacity Condenser fan

Boiler control

Step control

Figure 31.4 Electrical control diagram for small air-conditioningsystem

31.7 Commissioning of control systems

The setting up, testing and recording of all control functions of arefrigeration or air-conditioning system must be seen as part of thecommissioning procedure It requires that all items of equipmentwithin the system are in working order and that the function ofeach item of control is checked, initially set at the design value (if

this is known), readjusted as necessary during the testing stages,and finally placed on record as part of the commissioningdocumentation.

Most controllers have adjustments, not only to the set points but

to differentials, time delays and response rates It is of paramountimportance that these are set up by an engineer who completelyunderstands their function Such settings should be marked on theinstrument itself and recorded separately, since unauthorized personsmay later upset these adjustments [63]

32 Commissioning

32.1 Specification

The commissioning of a refrigeration or air-conditioning plant startsfrom the stage of static completion and progresses through thesetting-to-work procedure and regulation to a state of full workingorder to specified requirements

Commissioning is the completion stage of a contract, when thecontractor considers that the plant is in a correct state to hand overand the purchaser considers that it is in a correct state to accept andpay for

Since the final object of commissioning is to ensure that theequipment meets with a specified set of conditions, this specificationmust be clearly stated and, hopefully, would have been clearly statedwhen the contract was placed

A contract should state the following:

1 The medium or product to be cooled, or the area to be cooled

2 The total required cooling capacity, or mass throughput ofproduct with ingoing and outgoing temperatures

3 The required limits of control

4 A realistic ambient condition for condenser water or air, andfor fresh air supply

– 10°C in a coldroom measuring X × Y × Z and insulated with

100 mm expanded polystyrene, and freeze 20 t/day of chilled beefentering at 0°C, assuming an ambient air temperature of 26°C

Example 32.2 The plant is to cool water at the rate of 120 litre/sfrom 18°C to 4°C The ambient wet bulb temperature is 19°C

Example 32.3 The plant is to have a capacity of 325 kW when

cooling a 30% aqueous solution of propylene glycol from – 4°C to– 7.2°C Water is available from the main cooling tower at 23°C.

Example 32.4 The direct expansion coil is to cool 6.7 m3/s airfrom 21°C dry bulb, 50% saturation, to 10°C dry bulb, 85% saturation,when evaporating at 4°C

Example 32.5 The air-conditioner is to maintain 23°C dry bulbplus or minus 0.75°C and 50% saturation plus or minus 4%, in the

room shown on drawing XYZ, assuming an internal load of 28 kW,

including the four occupants The maximum ambient conditionsare 28°C dry bulb, 19.5°C wet bulb

If no such specification exists at the time of commissioning, somebasis of acceptance must be agreed between the parties concerned.Basic flow diagrams should be available and, if not, the com-missioning engineer must draw them up, against which actual plantperformance can be checked (See Figure 32.1.)

VSV 51 evaporation –12.6 ° C Chiller No 2 –1.3 ° C

VSV 51 evaporation –10 ° C Chiller No 1

+1.6 ° C

150 m3/h 41.7 litre/s

480 kPa Glycol pump

Glycol circuit

Balance tank

Figure 32.1 Basic flow diagram for liquid chilling process plant

The commissioning engineer will require details and ratings ofall major items of the plant and copies of any manufacturer’sinstructions on setting to work and operating their products If thisinformation is not to hand, the work will be delayed

1 A comprehensive mechanical inspection to confirm that allcomponents are to the agreed specification.

2 Check all wiring and electrical controls, for correct circuitry,security of terminals, continuity, insulation, compliance with safetyregulations such as IEE, marking of terminals and cables, etc

3 Check action of all controls as far as may be possible withoutrunning any item [63]

4 Check all water systems filled Start pumps and check rotation,flows, and pressures [64]

5 Start fans; check rotation, flows, and pressures [65]

6 Balance duct and grille flows [65]

7 Start main refrigeration system [34] Allow to run on load untilsteady conditions are reached

8 Set automatic controls to their approximate values, so the systemwill run without attention

The services of specialist personnel and plant mechanics will berequired during this period to operate the equipment and carryout any day-to-day attention Care should be taken that this workdoes not come into conflict with the recommendations of suppliers,

or invalidate their warranties Where major items have not yet beenaccepted from suppliers, it will be advisable to retain their owncommissioning engineer or other attendant until the project iscomplete All necessary maintenance must be carried out, sinceany premature failure of a component may be blamed on such anomission

The whole system is now left to run for a shake-down period,which may be from a few hours to several days, depending on thesize and complexity During this time, all components will be checkedfor vibration, leaks or other malfunction, and remedial action taken.Low-temperature systems and cold stores should be brought downslowly, to allow for shrinkage in the structure A fall of 5 K per day

is reasonable, moving more slowly through the band + 2°C to – 2°C

At the end of the shake-down period all strainers and filters arecleaned ready for the final test If compressor oil is seen to becontaminated, this should be changed (see Section 11.10)

32.4 Calibration and final commissioning

Final adjustments should now be made to the following, and anyother items of this sort:

1 Air flows, by setting of dampers This entails measurement atvarious points and comparison with the design figures

2 Concentration of any brines present

3 Water and other liquid flows

4 Starter overloads and the settings of safety controls such as sure cut-outs and safety thermostats

pres-In the final commissioning stage, readings are taken and recorded

of all measurable quantities in the system, and compared with thespecification and design figures The following, as applicable, should

be considered as the absolute minimum to be taken and recorded:

1 Ambient conditions, dry and wet bulbs

2 All fluid flows, temperatures, and pump, fan and filter pressures

3 Refrigerant pressures and temperatures at expansion valve inlet,evaporator outlet, and compressor suction and discharge

4 Settings of all adjustable controls

5 Electric motor currents

It is probable that a full load cannot be obtained during the finaltest, for reasons of low ambient or lack of completion of otherequipment for the process In such circumstances, the commissioningengineer must establish the load which prevailed at the time andmake an estimate of the system performance, on the basis of timerun, or otherwise interpret the figures obtained In such cases itmay be advisable to agree to a tentative acceptance of the plant andcarry out a full-load test at a later date

Errors may come to light during this work and be possible tocorrect If not, the acceptance or otherwise will be matter fornegotiation between contractor and customer If no agreement can

be reached, it may be necessary to refer the commissioning procedure

to arbitration

32.5 Commissioning records

The commissioning log, which is the basis of the handover andacceptance of the system, should have as much of the original designdata as is available, cross-checked against readings taken during thefinal tests Line flow diagrams, if not already supplied, should beprepared and kept with the final drawings and wiring and othercontrol diagrams Details of the information which should be gathered

1 A copy of the commissioning log.

2 Flow, control, electrical and layout diagram and drawings

3 Operating instructions It is usual to instruct the user’s operators

as part of these final works

4 Copies of instructions and manuals for all proprietary items ofequipment

5 Maintenance instructions

It must be particularly noted that details should be entered atthis time of ambient and load conditions, and any other factorswhich have an interface outside the plant itself In this way, therelationship between ambient, load and plant can be established as

a guide to future seasonal and load variations It is helpful to theuser if some indication can be given of operating conditions underlight load, since the plant may well work most of its life at less thanfull load and the operators might not be able to interpret the readingstaken

Once this initial data has been recorded, a running log will indicatethe performance under service conditions As the load and ambientconditions change, the plant operators will be able to monitor theday-to-day conditions This establishes normal running Only by aclear understanding of what is normal can the abnormal be detected

33 Operation Maintenance.

Service Fault-finding.

Training

33.1 Operation

A large proportion of refrigerating and air-conditioning equipment

is now fully automatic in operation, but that does not absolve theuser from the responsibility of understanding how it operates andbeing able to observe this operation If this is not done, or if there

is no other way of monitoring performance, the plant may runabnormally for some time before a fault is noticed, by which timeconsiderable damage may have occurred

The initial requirement is for the equipment to be fitted withsufficient pressure gauges and other monitoring devices to indicatethe conditions under which it is working It is helpful to mark thesewith the normal working limits when commissioning the plant.Persons operating the plant should understand the meaning ofany indicator or warning lights fitted to the control panels It isimportant that the operator should be aware of the temperaturegradients to be expected with the system so as to be able to compareactual working conditions with the design figures Any changes should

be interpreted as changes of ambient or load A running log should

be kept, as far as possible, to monitor working conditions

Where the switching of plant is purely manual, the plantinstructions should specify the limits of control, and not leave these

to the shift operator, who may not be sufficiently skilled to take thecorrect decisions Standby plant is often fitted, and it is part of theoperation discipline to change over machines to ensure that theyget even wear and keep all sets in running order All operation staffshould be aware of the method of bringing standby plant into use

in an emergency Where refrigerant valves need to be opened or