Refrigeration and Air Conditioning 3 E Part 5 ppt

Figure 9.8 Seal cap shut-off valve Small valves which are to be operated frequently have a packlessgland, either a diaphragm or bellows, and a handwheel.. High-pressure cut-out Evaporato

A safer circuit injects the discharge gas directly after the expansionvalve or into the evaporator outlet and before the sensor of theexpansion valve With this arrangement, the expansion valve willadmit extra refrigerant, and gas entering the compressor will benormally cool These control methods are wasteful of energy.

9.12 Relief valves

Under several possible conditions of malfunction, high pressurescan occur in parts of the system and mechanical relief devices areadvised or mandatory The standard form of relief valve is a spring-loaded plunger valve No shut-off valve is permitted between therelief valve and the vessel it protects, unless two such valves arefitted, when the shut-off may isolate one at a time [13] Two valvesare required on a vessel greater than 285 litres in volume

In all cases, the outlet of the valve must be led to the open air, in

a location where the sudden discharge of refrigerant will not causeannoyance or danger Under certain circumstances, a relief valvefrom the high-pressure side may enter the low side of the samesystem Small vessels may have a plug of a low melting point metal,which will melt and release the pressure in the event of fire Plunger-type relief valves, if located outdoors, should be protected from theingress of rain, which may corrode the seat Steel valves, wheninstalled, should have a little oil poured in to cover the seat as rustprotection

To prevent overpressure within a compressor, a relief valve orbursting disc is often fitted between the inlet and dischargeconnections

valve

Figure 9.7 Capacity reduction by hot gas injection, with compensatingliquid injection

Controls and other circuit components 115

9.13 Shut-off valves

Manual stop valves are required throughout a circuit to permitisolation during partial operation, service or maintenance (see Figure9.8)

Figure 9.8 Seal cap shut-off valve

Small valves which are to be operated frequently have a packlessgland, either a diaphragm or bellows, and a handwheel

Valves of all sizes which are only used occasionally will be sealedwith ‘O’ rings As a safeguard against leakage, they have no handwheelfitted and the stem is provided with a covering cap which is onlyremoved when the valve is to be operated The stem will have flatsfor operation by a spanner Most such valves can be back-seated topermit changing the ‘O’ rings

Valves should not be installed with the stem downwards, as anyinternal dirt will fall into the spindle thread.

Under low-temperature conditions, ice will form on the spindleand will be forced into the gland if the valve is operated quickly.Under such circumstances, the spindle should be well greased, orthe ice melted off first

Ser vice stop valves on small compressors may also carr y aconnection for a pressure cut-out or gauge, or for the temporaryfitting of guages or charging lines when servicing The valve back-seats to close off this port while gauges are being fitted Valve seatsare commonly of soft metal or of a resistant plastic such as PTFE

9.14 Strainers

Piping circuits will usually contain a small quantity of dirt, scale andswarf, no matter what care is taken to keep these out A strainer isfitted in the compressor suction to trap such particles before theycan enter the machine Such strainers are of metal mesh and will belocated where they can be removed for cleaning In some con-figurations two strainers may be fitted

As an extra safeguard, on new compressors a fabric liner may befitted inside the mesh strainer to catch fine dirt which will be present.Such liners must be removed at the end of the running-in period,

as they create a high resistance to gas flow

Oil strainers may be of metal mesh and within the sump, in whichcase the sump must be opened for cleaning Self-cleaning disc strainersare also used, the dirt falling into a drain pot or into the sump itself.There is an increasing tendency to provide replaceable fabric oilfilters external to the compressor body, following automobile practice

9.15 Strainer-driers

With the halocarbons, it is essential to reduce the water content ofthe refrigerant circuit to a minimum by careful drying of componentsand the fitting of drying agents in the system The common form ofdrier is a capsule charged with a solid desiccant such as silica gel,activated alumina or zeolite (molecular sieve), and located in theliquid line ahead of the expansion valve These capsules must havestrainers to prevent loss of the drying agent into the circuit, and soform an effective strainer-drier to also protect the valve orifice fromdamage by fine debris

Large driers are made so they can be opened, and the spentdrying agent removed and replaced with new Small sizes arethrowaway Driers may also be used in the suction line

Controls and other circuit components 117

9.16 Sight glasses

Pipeline sight glasses can be used to indicate whether gas is present

in a pipe which sould be carrying only liquid The main application

in refrigeration is in the liquid line from the receiver to the expansionvalve If the equipment is running correctly, only liquid will bepresent and any gas bubbles seen will indicate a refrigerant shortage(see also Chapters 11 and 33)

Sight glasses for the halocarbons are commonly made of brass,and may have solder or flare connections For ammonia, they aremade of steel or cast iron

Since the interior of the system can be seen at this point, advantage

is taken in most types to insert a moisture-sensitive chemical whichwill indicate an excess of water by a change of colour When such anindication is seen, the drier needs changing or recharging, and thecolour should then revert to the ‘dry’ shade

9.17 Charging connection

In order to admit the initial refrigerant charge into the circuit, oradd further if required, a charging connection is required Thesafest place to introduce refrigerant will be ahead of the expansionvalve, which can then control the flow and prevent liquid reachingthe compressor The usual position is in a branch of the liquid line,and it is fitted with a shut-off valve and a suitable connector with asealing cap or flange A valve is needed in the main liquid line, justupstream from the branch and within reach For the method ofuse, see Chapter 11

The relative positions of all these components are shown in thecomplete circuit in Figure 9.9

9.18 Auxiliary components

More complex refrigeration systems may have components for specificpurposes which are not encountered in simple circuits Non-return

or check [24] valves will be found in the following positions:

1 On heat pump circuits, to prevent flow through expansion valveswhich are not in service on one cycle

2 On hot gas circuits, to prevent the gas entering another evaporator

3 Where several compressors discharge into one condenser, toprevent liquid condensing back to an idle compressor

4 Where two or more evaporators work at different pressures, toprevent suction gas flowing back to the colder ones

9.19 Liquid refrigerant pumps

In a flooded evaporator, the movement of the liquid may be sluggish,with resulting low heat transfer Liquid pumps can be used to circulaterefrigerant from the suction separator (or ‘surge drum’), throughthe evaporator(s) and back In the separator, remaining liquid fallsback and is recirculated, while vapour goes to the compressor (seeFigure 9.10) These pumps are found mainly on low-temperaturecoldrooms, blast freezers and process applications [25]

High-pressure cut-out

Evaporator

Oil pressure safety switch Oil pressure gauge Compressor

Expansion

valve

Low-pressure cut-out Equalizer

Phial Suctionstop valve

Suction pressure gauge

Discharge pressure gauge

Sight glass Charging connection

Solenoid

valve

Strainer drier

Valve Receiver

outlet valve

Receiver

Relief

Level gauge

Discharge stop valve

Condenser

Water

Condensing pressure control

Figure 9.9 Dry expansion circuit showing components

Suction

Liquid

Low-pressure accumulator –separator

Evaporator 1

Evaporator 2

Evaporator 3

Liquid refrigerant pump

Pumped liquid supply main

Figure 9.10 Pumped liquid circuit

9.20 Suction separators

Suction line accumulators are sometimes inserted in halocarbon circuits,

to serve the same purpose of separating return liquid and prevent

it passing over to the compressor Since this liquid will be carrying

Controls and other circuit components 119

oil, and this oil must be returned to the compressor, the outlet pipewithin the separator dips to the bottom of this vessel and has a smallbleed hole, to suck the oil out (see Figure 9.11)

Suction traps are now widely used, particularly on rolling piston

and scroll compressors, to prevent liquid passing into the compressor

9.21 Liquid separators

Separation vessels can be inserted in a liquid line Liquid will fall tothe bottom and pass through an expansion device to an evaporator.High pressure gas will rise to the top of the vessel and can then beused for heating or for hot gas defrost of another heat exchanger

9.22 Overheat protection

Small compressors will have motor overheat protection adjacent tothe hermetic shell or built into the winding (see Section 4.8) andlarger motors will have contactor–starters with overcurrent devices.Overheat protection is also fitted on many machines, to guard againsthigh motor winding, cylinder head or oil temperatures These usuallytake the form of thermistor detectors, connected to stop the motor

Accumulator body

Oil return bleed hole

Figure 9.11 Suction line accumulator or liquid trap

9.23 Integrated control systems

The purpose of the various electromechanical elements of a circuit

is to effect monitoring, safety and automatic control, and these may

be connected separately into a custom-built system The availability

of electronic logic circuits gives the possibility of integrated systemsand superior control, using a large number of input signals Observedparameters are:

Electrical supply

Load temperature

Air and water flows

Number of compressors running, and loading stages

Condenser pressure

Number of condensers running

Condenser fan speed

Control may then be effected of:

Number and stages of compressors running

Limitation of motor start frequency

Limitation of maximum electrical demand

Number of fans or condensers running

Fan speeds

Number of fans or evaporators running

Warning of faulty plant

Shutting down faulty plant

Starting standby plant

Monitoring energy used

Printed running logs

Scheduling maintenance

Remote alarm systems

Integrated control systems are mainly found on factory-assembledequipment, but the increased use of programmable logic controllersfor process control is giving designers and installation mechanicsthe experience to apply these methods to custom-built refrigerationsystems [26]

10 Selection and balancing of

components

10.1 Balanced system design

The four main components of a vapour compression refrigerationcircuit – the evaporator, the compressor, the condenser and theexpansion valve – must be selected to give a balanced system.Each of these items must:

1 Be suitable for the application

2 Be correctly sized for the duty

3 Function as required in conjunction with the other componentsThe system designer must consider these components and examinethe options which may be available in order to determine a bestselection with reference to first cost, installation, operation, runningcost, maintenance and expected life The following factors are some

of those affecting the final decision:

1 If the initial capital cost is the deciding factor, then the plantwill almost certainly be more expensive to operate

2 Installation of a new plant may cause serious disruption of theuser’s ongoing business, and the extent of this disruption should

be determined before it is too late Apart from the installation

of the plant itself, there is the associated builders’ work and thetemporary disconnection of other services The use of factory-built packaged equipment helps to reduce this nuisance

3 Most systems are now automatic in operation, but the user’sstaff must be aware of the control system and have facilities torun on manual control, as far as this may be possible, in theevent of a control failure

4 Operators must understand the function of the system If not,they will not have the confidence to work on or with it, and the

plant will not be operated at its best efficiency Also, if it breaksdown for any reason, they will be unable to put it right.

5 The cost of electricity, other fuels, water, spare parts and operatingand maintenance labour represents the greater part of the owningcosts of a refrigeration system It is probable that a small extraexpenditure on some items, especially heat exchangers, willreduce running costs

6 A lot of modern equipment is almost maintenance-free but theuser must be aware of what maintenance functions are required,whether these are within the scope of his own staff and where toget assistance Where maintenance is contracted out, it isimportant that this should be carried out, at least for the warrantyperiod, by the supplier

7 Life expectancies are 15–20 years for refrigeration systems, andsomewhat less for small packaged equipment Where the need

is for a shorter period, such as a limited production run or for

a temporary building, equipment of lower quality or hand plant could be considered

second-10.2 Evaporating temperature

The next step is to decide a suitable evaporating temperature Thiswill be set by the required load condition and the appropriatetemperature differential (∆T) across the evaporator In the context

of evaporator selection, the ∆T used is the difference between the

evaporating refrigerant and the temperature of the fluid enteringthe cooler, not the log mean temperature difference (see [1–5])

In systems where the evaporator cools air, the air itself becomesthe heat transfer medium and its temperature and humidity must

be considered in relation to the end product Where the productcannot suffer dehydration, the ∆T may be high, so as to reduce the

size and cost of the coil, but the lower the evaporating temperaturefalls, the lower will be the capacity of the compressor and its COP

In these circumstances, a first estimate might be taken with a ∆T of

10–12 K and cross-checked with alternative plant either side of thisrange In each case, the ‘owning’ cost, i.e taking into account therunning costs, should be considered by the user For a cold storeexample, running 8760 hours per year, see Table 10.1

Unsealed products will be affected by low humidity of the air inthe cooled space and may suffer dehydration Conversely, somefood products such as fresh meat will deteriorate in high humidities.Since the dew point of the air approaches the fin surface temperature

of the evaporator (see also Chapter 24), the inside humidity is afunction of the coil ∆T That is to say, the colder the fin surface, the

Selection and balancing of components 123

more moisture it will condense out of the air, and the lower will bethe humidity within the space Optimum conditions for all productslikely to be stored in cooled atmospheres will be found in the standardreference books, or may be known from trade practice The followingmay be taken as a guide:

Products that dehydrate quickly, such as

Products requiring about 85% saturated air ∆T = 6 K

Products requiring 80% saturation or drier ∆T = 8 K

Materials not sensitive to dehydration ∆T = 10 K upwards

A further consideration may be the possibility of reducing ice

build-up on the evaporator, whether this is in the form of frost on fins orice on the coils of a liquid chilling coil Where temperatures close

to freezing point are required, it may be an advantage to designwith an evaporator temperature high enough to avoid frost or ice –either for safety or to simplify the defrost method

10.3 Evaporator

Once the evaporating temperature has been provisionally decided,

an evaporator can be selected from catalogue data or designed forthe purpose Catalogue ratings are usually in the form of coolingcapacity for a given temperature difference between the enteringfluid and the evaporating refrigerant, since the user cannot easilydetermine the ln MTD Units will be in kW/K (Btu/(h °F) or kcal/(h °C) in old catalogues)

This factor, the basic rating, is assumed constant throughout the

design working range of the cooler and this approximation is goodenough for equipment selection The basic rating will change withfluid mass flow and, to a lesser extent, with working temperature Itmay change drastically with fluids such as the glycol brines, sincethe viscosity and hence the convection heat transfer factor alter at

lower temperatures In unusual applications, the supplier should

be consulted (See also Section 35.4.)

10.4 Compressor

The choice of compressor type is now a wide one, and at least twoalternatives should be considered before making a final selection.Compressor capacities may be shown in tables or curves, and will

be for a given refrigerant and a range of condensing pressures (seeSection 4.13) They may also show the power taken At this stage, afirst guess must be taken for the condensing temperature, and thismight be 15K above the summer dry bulb for an air-cooled condenser

or 12 K above the wet bulb temperature in the case of water orevaporative cooling

The balance condition between the evaporator and the compressorcan be visualized in a graphic solution, superimposing the basicrating of the cooler on the compressor curves (see Figure 10.1).While it is usual to consider only the balance at the maximumsummer ambient, the application engineer should be aware of therunning conditions in cooler weather If this is not favourable to

– 25

25 ° C

40 ° C

Condensing temperature

Room temperature

Evaporating temperature ( ° C) Cooler rating

Figure 10.1 Balance condition between compressor and evaporator

Selection and balancing of components 125

the product, some average choice may be made, or a back pressurevalve inserted to prevent the evaporating temperature droppingtoo low (see Figure 10.2) A different set of conditions will alsooccur if the compressor has capacity control If this is likely to causeproblems, then a compressor with 50% capacity control may beconnected to two equal evaporators, and one of these shut off athalf load

Condenser

Compressor Back pressure

regulating valve

Evaporator

Expansion valve

Figure 10.2 Use of back pressure regulating valve to maintainevaporator pressure (and temperature)

10.5 Condenser

A first guess of a condensing temperature has already been taken as

a rough guide Users should be aware of the wide difference inowning costs arising from the choice of condenser, so the optionsshould be compared The buyer who is influenced only by first costwill almost certainly face higher fuel bills Certain machines, such

as the centrifugal compressor, are very sensitive to high condensingconditions, and the correct choice (in this case, of a cooling tower)can give a considerable gain in COP

Users seeking tender quotations should demand relative runningcosts and make their decision on the basis of their anticipated runningtimes and so of the expected fuel costs, taking into account the slowinflation of electricity tariffs Buyers must be aware of the tendency

of a contractor to offer only one make or type of equipment, andwhere this situation arises, alternative tenders should be sought

In most climates the wet bulb temperature is well below the drybulb temperature and there is an advantage in using water orevaporative cooling for larger plant These options need to beinvestigated and compared The present concern over spray-bornediseases may indicate a preference for air cooling in the vicinity ofinstitutions but correct maintenance of water cooling towers andevaporative condensers will permit their use elsewhere Table 10.2,based on the tentative temperature differences of 15K and 12K

given above, shows that such figures need to be reconsidered inextreme cases For example, if it is necessary to use an air-cooledcondenser in the desert, because there is no water available, thenthere will be considerable economy in oversizing the condenser toreduce the condensing temperature from a first guess of 62°C down

to, possibly, 56°C

Some manufacturers of air-cooled packaged condensing unitsoffer a range of condenser sizes for each compressor, and theseshould be closely compared in terms of higher duty and lowerrunning costs

The maximum design condensing temperature will only applywhen the ambient is at its hottest, and full advantage should always

be taken to allow this temperature to drop at cooler times, down toits minimum working limit (see also [8–10]) Systems should beallowed to drop to a condensing temperature of 25°C when thecooling medium permits this, and some systems can go a lot lower

A true estimate of owning cost should take this into account.The performance of alternative condensers with a compressor–evaporator system can be shown graphically but the curves will have

to be plotted, since manufacturers cannot be expected to supplythese figures for all conditions of working In this construction(Figure 10.3) the rating curves are the rejected heat from thecompressor, i.e cooling duty plus compressor power These areplotted against the basic rating of the condenser Some condensermanufacturers provide rating curves based on the cooling capacity

of the compressor and using typical factors for the power (see Example6.2)

Air-cooled condensers require a large air flow for a given heatrejection duty and the limitation on their use is reached on account

of their size and the need to get enough air Water or evaporativecooling should always be considered as a possibility, except for smallersizes or where using packaged condensing units

Table 10.2

Selection and balancing of components 127

10.6 Expansion valve

The expansion valve is a passive orifice, through which the liquidrefrigerant is forced by the pressure difference between thecondensing and evaporating conditions Capacity ratings are given

in the catalogues of manufacturers and suppliers Types in generaluse are:

1 Capillary tubes, for small hermetic systems These are factoryselected and cannot be adjusted

2 Solenoid valves with liquid level sensors or liquid level valves formost flooded evaporators

3 High-pressure float valves plus handset throttle valves for someflooded and low-pressure receiver circuits

4 Thermostatic expansion valves or electronic expansion valvesfor most dry expansion circuits

Troubles arise with the selection of thermostatic expansion valves,since this is the type generally used in custom-built systems and, forthese, selected outside a factory

It is usual to select a thermostatic expansion valve for the maximumduty and at the summer condensing condition, taking into accountthe pressure drop through a liquid distributor in the case of amultiple-feed coil Valve ratings are given for a range of pressuredifferences, i.e for a range of condensing conditions, in Table 10.3.

It might be thought that the duty varies with pressure differenceaccording to fluid flow laws, but this valve capacity is plotted againstthe expected mass flow curve in Figure 10.4 It is seen that the valvecapacity is greater This is because the refrigerant can absorb moreheat if it is colder on entry This means that the valve may be able

to pass the required amount of liquid at a much lower condensingpressure Conversely, if the valve is selected at a lower pressuredifference (possibly corresponding to a condensing condition inthe UK of 20–25°C), the valve will not be grossly oversized at themaximum summer condition

Rated valve duty Fluid flow law

Pressure difference (bar)

Figure 10.4 Rating curve for expansion valve

A growing awareness of energy economy is leading to more carefulapplication of this component Suppliers are ready to help withadvice and optimum selections