Refrigeration and Air Conditioning 3E Episode 4 pot

A number of tubes is omitted at the top of the shell to give space forthe suction gas to escape clear of the surface without entraining Refrigerant liquid level c Figure 7.1 Flooded evap

etc., will have finned pipe coils (see Figure 7.2) In all but very smallcoolers, there will be fans to blow the air over the coil.

Construction materials will be the same as for air-cooledcondensers Aluminium fins on copper tube are the most commonfor the halocarbons, with stainless steel or aluminium tube forammonia Frost or condensed water will form on the fin surfaceand must be drained away To permit this, fins will be vertical andthe air flow horizontal, with a drain tray provided under

The size of the tube will be such that the velocity of the boilingfluid within it will cause turbulence to promote heat transfer Tubediameters will vary from 9 mm to 32 mm, according to the size ofcoil

Fin spacing will be a compromise between compactness (andcost) and the tendency for the interfin spaces to block with condensedmoisture or frost Spacings will vary from 2 mm on a compact air-conditioner to 12 mm on a low-temperature coldroom coil [8]

7.4 Liquid cooling evaporators

Liquid cooling is mostly in shell-and-tube or shell-and-coil evaporators

In the shell-and-tube type, the liquid is usually in the pipes andthe shell is some three-quarters full of the liquid, boiling refrigerant

A number of tubes is omitted at the top of the shell to give space forthe suction gas to escape clear of the surface without entraining

Refrigerant liquid level

(c)

Figure 7.1 Flooded evaporators (a) Shell-and-tube (b) Jacketted.(c) Raceway

(b)

Operated in this manner, the shell-and-tube type is a floodedevaporator (see Figure 7.3) and has oil drainage pots if usingammonia, or a mixture bleed system if the refrigerant is one of thehalocarbons The speed of the liquid within the tubes should beabout 1 m/s or more, to promote internal turbulence for goodheat transfer End cover baffles will constrain the flow to a number

of passes, as with the shell-and-tube condenser (See Section 6.4.)

Liquid in

Suction

Water in

Water out

Figure 7.3 Shell-and-tube evaporator, flooded

Evaporators of this general type with dry expansion circuits willhave the refrigerant within the tubes, in order to maintain a suitablecontinuous velocity for oil transport, and the liquid in the shell.These can be made as shell-and-tube, with the refrigerant constrained

to a number of passes, or may be shell-and-coil (see Figure 7.4) Inboth these configurations, baffles are needed on the water side toimprove the turbulence, and the tubes may be finned on the outside.Internal swirl strips or wires will help to keep liquid refrigerant incontact with the tube wall

Figure 7.4 Shell-and-coil evaporator

Suction

Water out

Water in Refrigerant in

Evaporators 87

Liquid cooling evaporators may comprise a pipe coil in an opentank, and can have flooded or dry expansion circuitry Floodedcoils will be connected to a combined liquid accumulator and suctionseparator (usually termed the surge drum), in the form of a horizontal

or vertical drum (see Figures 7.1c and 7.5) The expansion valvemaintains a liquid level in this drum and a natural circulation is set

up by the bubbles escaping from the liquid refrigerant at the heatexchanger surface Dry expansion coils for immersion in an opentank will be in a continuous circuit or a number of parallel circuits(see Figure 7.6) Liquid velocity over such coils can be increased bytank baffles and there may be special purpose agitators, as in an ice-making tank (see Figure 12.1) Coils within an open tank can beallowed to collect a layer of ice during off-load periods, thus providingthermal storage and giving a reserve of cooling capacity at peakload times (see also Chapter 12)

Suction

Refrigerant

liquid level

Refrigerant in

Fluid level in tank

Figure 7.5 Flooded tank evaporator

Another type comprises a bank of corrugated plates, formingalternative paths for refrigerant and liquid, similar to that shown inFigure 17.1, of brazed or welded construction

Where water is to be cooled close to its freezing point withoutrisk of damage to the evaporator, the latter is commonly arrangedabove the water-collection tank and a thin film of water runs overthe tubes Heat transfer is very high with a thin moving film ofliquid and, if any ice forms, it will be on the outside, free to expand,and it will not damage the tube Such an evaporator is termed a

Baudelot cooler (Figure 7.7) It may be open, enclosed in dust-tight

Figure 7.6 Dry expansion tank evaporator (a) Section (b) Elevation

Vertical evaporator plates

Water tank

Circulating pump

Figure 7.7 Baudelot cooler

shields to avoid contamination of the product (as in surface milkand cream coolers), or may be enclosed in a pressure vessel as inthe Mojonniér cooler for soft drinks, which pressurizes with carbondioxide at the same time (Figure 17.5)

(a)

Baffles

(b)

Water out

Water in Refrigerant connections

Evaporators 89

Some liquids, such as vegetable fats and ice-cream mixes, increaseconsiderably in viscosity as they are cooled, sticking to the heatexchanger surface Evaporators for this duty are arranged in the

form of a hollow drum (see Figure 7.1b) surrounded by the refrigerant

and having internal rotating blades which scrape the product off as

it thickens, presenting a clean surface to the flow of product andimpelling the cold paste towards the outlet

7.5 Plate evaporators

Plate evaporators (Figure 7.8) are formed by cladding a tubular coilwith sheet metal, welding together two embossed plates, or fromaluminium extrusions

The extended flat face may be used for air cooling, for liquidcooling if immersed in a tank, or as a Baudelot cooler

The major use for flat plate evaporators is to cool a solid product

by conduction, the product being formed in rectangular packagesand held close between a pair of adjacent plates

In the horizontal plate freezer (Figure 7.9a), the plates are arranged

in a stack on slides, so that the intermediate spaces can be openedand closed Trays, boxes or cartons of the product are loaded betweenthe plates and the stack is closed to give good contact on both sides.When the necessary cooling is complete, the plates are opened andthe product removed

The vertical plate freezer (Figure 7.9b) is used to form solid

blocks of a wet product, typically fish When frozen solid, the surfacesare thawed and the blocks pushed up and out of the bank

To ensure good heat transfer on the inner surface of the platesand achieve a high rate of usage, liquid refrigerant is circulated by

a pump at a rate 5–12 times the rate of evaporation

If a plate evaporator is partially filled with brine (see Figure 7.8d)

this can be frozen down while the plate is on light load, and thereserve of cooling capacity used at other times The freezing point

of the brine can be formulated according to the particular applicationand the plate can be made as thick as may be required for thethermal storage needed The major application of this device is thecooling of vehicles The plates are frozen down at night, or othertimes when the vehicle is not in use, and the frozen brine keeps thesurface of the plate cold while the vehicle is on the road Therefrigeration machinery may be on the vehicle or static

7.6 Defrosting

Air cooling evaporators working below 0°C will accumulate frostwhich must be removed periodically, since it will obstruct heat transfer

Evaporators of suitable and robust construction can be defrosted

by brushing, scraping or chipping, but these methods are intensive and may lead to damage of the plant

Where the surrounding air is always at +4°C or higher, it will besufficient to stop the refrigerant for a period and allow the frost tomelt off (as in the auto-defrost domestic refrigerator) This methodcan be used for coldrooms, packaged air-conditioners etc., wherethe service period can be interrupted.

For lower temperatures, heat must be applied to melt the frostwithin a reasonable time and ensure that it drains away Methodsused are as follows:

1 Electric resistance heaters Elements are within the coil or directlyunder it

2 Hot gas A branch pipe from the compressor discharge feedssuperheated gas to the coil The compressor must still be working

on another evaporator to make hot gas available Heat storagecapsules can be built into the circuit to provide a limited reserve

of heat for a small installation

3 Reverse cycle The direction of flow of the refrigerant is reversed

to make the evaporator act as a condenser Heat storage oranother evaporator are needed as a heat source

In each of these cases, arrangements must be made to remove coldrefrigerant from the coil while defrosting is in progress Drip traysand drain pipes may require supplementary heating

Condensed water will run down the evaporator fins to a collectiontray below the coil From there, drain pipes will take this water to adrain If plastic pipe is used, it should be black to exclude daylight,

or slime will grow inside the tube Drain pipes passing throughrooms below freezing point need to be fitted with trace heaters.Where the outlet drain is higher than the coil, the water needs to

be pumped away for disposal Condensate pumps are fitted to liftthis water to drain by gravity Such pumps are usually of the peristaltictype

8 Expansion valves

8.1 General

The purpose of the expansion valve is to control the flow of refrigerantfrom the high-pressure condensing side of the system into the low-pressure evaporator In most cases, the pressure reduction is achievedthrough a variable flow orifice, either modulating or two-position.Expansion valves may be classified according to the method of control

8.2 Low-pressure float valves

Flooded evaporators require a constant liquid level, so that thetubes remain wetted A simple float valve suffices, but must be locatedwith the float outside the evaporator shell, since the surface of theboiling liquid is agitated and the constant movement would causeexcessive wear in the mechanism The float is therefore containedwithin a separate chamber, coupled with balance lines to the shell(see Figure 8.1)

Such a valve is a metering device and may not provide positiveshut-off when the compressor is stopped Under these circumstances,refrigerant will continue to leak into the evaporator until pressureshave equalized, and the liquid level might rise too close to thesuction outlet To provide this shut-off, a solenoid valve is needed

in the liquid line

8.3 Low-pressure float switches

Since the low-pressure float needs a solenoid valve for tight closure,this valve can be used as an on–off control in conjunction with apre-set orifice and controlled by a float switch (Figure 8.2).The commonest form of level detector is a metallic float carrying

an iron core which rises and falls within a sealing sleeve An inductioncoil surrounds the sleeve and is used to detect the position of the

Upper balance pipe Suction

High-pressure liquid

Low-pressure float

Solenoid valve Liquid level

Lower balance pipe

Suction

Liquid level

Controller

Figure 8.2 Low-pressure float switch

Figure 8.1 Low-pressure float valve on flooded cooler

Throttle valve

Solenoid valve

Expansion valves 95

core The resulting signal is amplified to switch the solenoid valve,and can be adjusted for level and sensitivity A throttle valve is fitted

to provide the pressure-reducing device

Should a float control fail, the level in the shell may rise andliquid pass into the compressor suction To warn of this, a secondfloat switch is usually fitted at a higher level, to operate an alarmand cut-out

Where a flooded coil is located in a liquid tank, the refrigerantlevel will be within the tank, making it difficult to position the levelcontrol In such cases, a gas trap or siphon can be formed in thelower balance pipe to give an indirect level in the float chamber.Siphons or traps can also be arranged to contain a non-volatilefluid such as oil, so that the balance pipes remain free from frost

8.4 High-pressure float valve

On a single-evaporator flooded system, a float valve can be fittedwhich will pass any drained liquid from the condenser direct to theevaporator The action is the same as that of a steam trap The floatchamber is at condenser pressure and the control is termed a high-pressure float (Figure 8.3)

Condenser

High-pressure liquid

High-pressure float expansion valve

Figure 8.3 High-pressure float valve

The refrigerant charge of such a system is critical, since it mustnot exceed the working capacity of the evaporator It is not possible

to have a receiver in circuit and this control cannot feed more thanone evaporator, since it cannot detect the needs of either

The difficulty of the critical charge can be overcome by allowingany surplus liquid refrigerant leaving the evaporator to spill over

into a receiver or accumulator in the suction line, and boiling thisoff with the warm liquid leaving the condenser In this system, the

low-pressure receiver circuit, liquid is drained from the condenser through

the high-pressure float, but the final step of pressure drop takesplace in a secondary expansion valve after the warm liquid haspassed through coils within the receiver In this way, heat is available

to boil off surplus liquid leaving the evaporator (see Figure 8.4).Two heat exchangers carry the warm liquid from the condenserwithin this vessel The first coil is in the upper part of the receiver,and provides enough superheat to ensure that gas enters thecompressor in a dry condition The lower coil boils off surplusliquid leaving the evaporator itself With this method of refrigerantfeed, the evaporator has a better internal wetted surface, with animprovement in heat transfer

Evaporator

Discharge

Suction

Wet return

Compressor

Condenser

Warm liquid High-pressure float valve

to drain condenser Low-pressure

receiver

Cooled liquid Expansion

valve

Cold liquid +

some flash gas

Figure 8.4 Low-pressure receiver circuit

The low-pressure receiver system can be adapted to compoundcompression and can be fitted with hot gas defrost by reverse gasflow In both circuits the low-pressure receiver provides the safetyvessel to prevent liquid entering the compressor Providing the high-pressure float is correctly sized, this system can operate at lowcondenser pressures, saving compressor energy in cool weather.Where the halocarbon refrigerants are used in this system, an oildistillation device is fitted, working on the same principle as shown

in Figure 5.2

8.5 Thermostatic level control

If a small heater element is placed at the required liquid level of aflooded evaporator, together with a heat-sensing element, then the

Expansion valves 97

latter will detect a greater temperature if liquid refrigerant is notpresent This signal can be used to operate a solenoid valve

8.6 Expansion valves for dry expansion circuits

The dry expansion circuit does not have a liquid level which can bedetected, and another type of signal must be used to control thevalve Dry expansion circuits must be designed and installed so thatthere is no risk of liquid refrigerant returning to the compressor

To ensure this state, extra heat exchange surface is added to thatneeded, in order to heat the dry saturated gas into the superheatregion The amount of superheat is usually of the order of 5 K.Expansion valves for such circuits embody a mechanism whichwill detect the superheat of this gas leaving the evaporator (Figure

8.5) Refrigerant boils in the evaporator at T e and p e, until it is all

vapour, and then superheats to a condition T s , p e, at which it passes

to the suction line A separate container of the same refrigerant at

temperature T s would have a pressure p s , and the difference p s – p e

is a signal directly related to the amount of superheat

The basic thermostatic expansion valve (Figure 8.6) has a detectorand power element, charged with the same refrigerant as in the

circuit The pressure p s generated in the phial by the superheatedgas passes through the capillary tube to the top of the diaphragm

An adjustable spring provides the balance of p s – p e at the diaphragm,and the valve stem is attached at the centre Should the superheatfall for any reason, there will be a risk of liquid reaching the

compressor The T s will decrease with a corresponding drop in p s.The forces on the diaphragm are now out of balance and the springwill start to close the valve

Conversely if the load on the evaporator increases, refrigerantwill evaporate earlier and there will be more superheat at the phial

position Then p s will increase and open the valve wider to meet thenew demand

The phial must be larger in capacity than the rest of the powerelement or the charge within it may all pass into the valve capsule

and tube, if these are colder If this happened, the phial at T s would

contain only vapour and would not respond to a position T s , p s on

Use can be made of this latter effect The power element can belimit charged so that all the refrigerant within it has vaporized by apredetermined temperature (commonly 0°C) Above this point,the pressure within it will follow the gas laws:

Superheat All liquid

evaporated

Superheating gas

Te, pe Ts, pe

Ts, psSuction

The slope of the T–p curve is not constant, so that a fixed spring

pressure will result in greater superheat at a higher operatingtemperature range To allow for this and provide a valve which can

be used through a wide range of applications, the phial may becharged with a mixture of two or more volatile fluids to modify thecharacteristic curve

Figure 8.5 Superheat sensor on dry expansion circuit