Refrigeration and Air Conditioning 3 E Part 6 pdf

The cooling effect of a central refrigerating system can bedistributed by a heat-transferring liquid or secondary refrigerant.Where the working temperatures are always above 0°C, such as

12 Liquid chillers Ice Brines.

Thermal storage

12.1 Distributed cooling

A building or process having a large number of separated coolingloads could have a refrigeration system for each of these loads Itwill usually be more convenient to concentrate the cooling into oneplant

The cooling effect of a central refrigerating system can bedistributed by a heat-transferring liquid or secondary refrigerant.Where the working temperatures are always above 0°C, such as inair-conditioning, water is commonly used At temperatures belowthis, non-freezing liquids are used

12.2 Liquid chillers

The preferred secondary refrigerant will be water, if this can possiblysatisfy the temperature requirement, i.e if the load temperature issufficiently above 0°C that water can be circulated without risk offreezing

The greatest demand for chilled water is in air-conditioning systems(see also Chapters 23–28) For this duty, water is required at atemperature usually not lower than 5°C and, for this purpose, theevaporator will be of the shell-and-tube type, operating with refrigeranttemperatures close to freezing point A very wide range of factory-built package chillers is available and models are mainly one-pieceunits with integral water-cooled condensers as shown in Figures4.18 and 13.2 Other types may have air-cooled or evaporativecondensers, and so require refrigerant pipe connections on site tothese condensers Sizes range from 14 kW to 35 000 kW, mostinstallations being within the range 100–1500 kW

At water temperatures close to freezing, and with evaporators

Liquid chillers Ice Brines Thermal storage 145

which are vulnerable to ice damage, it is important to have adequatesafety controls, to check the calibration of these frequently and toavoid interference by unauthorized persons Nearly all troubles frompackaged water chillers arise from a failure of safety controls Severaltypes of controls are in use, frequently three or more on the sameequipment, but there should never be less than two of them:

1 Water flow switch, to stop the machine if flow stops in the chilledwater circuit

2 Refrigerant low-pressure cut-out

3 Water outlet low-temperature cut-out

4 Back pressure regulation valve (see Section 9.8)

5 Hot gas bypass valve, to keep the evaporating temperature upabove freezing point

Most packaged water chillers are large enough to have capacitycontrol devices in the compressor The main control thermostatwill unload the compressor as the water temperature approaches alower safe limit, so as to keep the water as cold as possible withoutrisk of freeze damage

In all but the smallest installations, two or more chillers will beused, or one chiller with two separate circuits This arrangementgives some continuity of the service if one machine is off-line formaintenance or another reason, gives better control and provideseconomy of running when loads are light

If water is required below 5°C, the approach to freezing pointbrings considerable danger of ice formation and possible damage

to the evaporator Some closed systems are in use and have eitheroversize heat exchange surfaces or high-efficiency-type surfaces Inboth of these, the object is to improve heat transfer so that thesurface in contact with the water will never be cold enough to causeice layers to accumulate

12.3 Baudelot coolers and ice bank coils

Water can be cooled safely to near freezing point, using evaporatorswhich have the refrigerant inside, with space for ice to form on theoutside of the surface without causing damage Two types are used:

1 Baudelot coolers (see Figures 7.7 and 7.8a) The evaporator

stands above a collection tank, and the water runs down theoutside surface in a thin layer Evaporator construction can bepipe coils or embossed plates The latter are now usually ofstainless steel, to avoid corrosion troubles Evaporators may beflooded or dry expansion During operation, a Baudelot cooler

146 Refrigeration and Air-Conditioning

may sometimes build up a thin layer of ice, but this does nodamage to the evaporator, and should melt off again when theload changes

2 Pipe coils within a water tank (see Figures 7.1c, 7.5 and 7.6).

Both flooded and dry expansion evaporators are in use Water

is circulated by pumps and/or special agitators This type ofwater chiller may be operated without formation of ice, or icemay be allowed to accumulate intentionally (see below).Water chillers of these two types are not usually made as singlepackages with their condensing unit, owing to the bulk of the systemand subsequent difficulty of installation

12.4 Ice manufacture

Ice may be made and transported to where the cooling effect isrequired The refrigeration energy available in this way will be mainlyits latent heat of melting, 334 kJ/kg, as it changes back to water.Ice can be made as thin slivers on the surface of evaporatordrums, and removed mechanically when the correct thickness hasbeen formed Either the drum or the scraper may rotate This is acontinuous process and the ice flakes fall directly onto the product

or into a storage bin below the machine Smaller units are made aspackages with the bin integral and cooled by a few turns of thesuction line or by a separate evaporator Small pieces of ice can beformed in or on tubes or other prismatic shapes made as evaporatortubes, arranged vertically Water is pumped over the surface tofreeze to the thickness or shape required The tube is then switched

to ‘defrost’ and the moulded section thaws sufficiently to slide off,possibly being chopped into short pieces by a rotating cutter Themachine itself will be made as a package, and the smaller sizes willinclude the condensing unit

The manufacture of ice in large blocks is by the can method (seeFigure 12.1), where a number of mould cans, filled with water, areimmersed to just below the rim in a tank of refrigerated brine Thesmallest block made in this way is 25 kg and will freeze in 8–15 h,using brine at –11°C Blocks up to 150 kg are made by this method.When frozen, the moulds are lifted from the tank and slightly warmed

to release the ice block from the sides of the moulds, when they can

be tipped out Blocks may go into storage or for direct use.Where the available water has a high proportion of solids, thereare methods either of pretreating the water or, by agitating thewater in the centre of the block (which freezes last), of removingthe concentrated dirty water before it becomes solid The core isthen refilled with fresh water [30]

Liquid chillers Ice Brines Thermal storage 147

Block ice can be made from sea water but the central core cannot

be frozen

12.5 Brines

Where a secondary refrigerant fluid is to be circulated, and theworking temperatures are at or below 0°C, then some form of non-freeze mixture must be used These are collectively termed brines.Brines may be, as the name suggests, solutions of inorganic salts

in water, and the two in general use are sodium chloride and calciumchloride Of these, the former is compatible with most foodstuffsand can be used in direct contact or in circumstances where thebrine may come into contact with the product Calcium chloridehas an unpleasant taste and cannot be permitted to contaminatefoods

12.6 Physical properties

With any solution, there will be one concentration which remainsliquid until it reaches a freezing point, and then will freeze solid.This is the eutectic mixture, and its freezing point is the eutecticpoint of the solute (see Figure 12.2) At all other concentrations, as

Ice crane Ice moulds Ice tanks Agitator

Ice tip Filling tank

Submerging coil evaporator Thawing tank

Ice store

Suction separator

Control panel Compressors Liquid

receiver Ceiling mounted air cooler

To condenser

Figure 12.1 Can ice plant (Courtesy of Hall-Thermotank Products Ltd)

148 Refrigeration and Air-Conditioning

Liquid

Liquid + salt slush Liquid

+ ice – 21.1 ° C Eutectic point 23.3%

0 10 20 30 40 50 60 70 80 90 100

Calcium chloride (% by weight)

(b)

– 51 ° C Eutectic point 29.6%

Liquid + ice

Figure 12.2 Eutectic curves (a) Sodium chloride in water

(b) Calcium chloride in water

the solution is cooled it will reach a temperature where the excesswater or solute will crystallize out, to form a slushy suspension of

Liquid chillers Ice Brines Thermal storage 149

the solid in the liquid, until the eutectic point is reached, when itwill all freeze solid

For economy of cost, and to reduce the viscosity (and so improveheat transfer), solutions weaker than eutectic are normally used,provided there is no risk of freezing at the evaporator

In salt brines, the water may be considered as the heat transfermedium, since the specific heat capacity of the salt content is low(see Figure 12.3) The specific heat capacity of the brine will therefore

150 Refrigeration and Air-Conditioning

decrease as the salt concentration increases At the same time, thespecific mass will increase

Non-freezing solutions can also be based on organic fluids,principally the glycols, of which ethylene and propylene glycol are

in general use Where contact with food is possible, propylene glycol(see Figure 12.4) should be used

Liquid

Liquid + slush ice

Extremely viscous liquid

0 15 20 30 40 50 60 80 100 Propylene glycol (% by weight)

Liquid chillers Ice Brines Thermal storage 151

The concentration of a solute has a considerable effect on theviscosity of the fluid and so on the surface convective resistance toheat flow There is little published data on these effects, so applicationsneed to be checked from basic principles Industrial alcohol(comprising ethyl alcohol with a statutory addition of methyl alcohol

to render it poisonous) may be used as a secondary refrigerant,either at 100% concentration or mixed with water The fluid has alow viscosity and good heat transfer, but is now little used on account

of its toxicity and the fire risk in high concentrations Other freeze heat transfer fluids are used in specialist trades

non-12.7 Brine circuits

Brine may be pumped to each cooling device, and the flow controlled

by means of shut-off or bypass valves to maintain the correcttemperature (see Figure 12.5)

Where a brine system services a multiple-temperature installationsuch as a range of food stores, the coolant may be too cold for someconditions, causing excessive dehydration of the product In suchcases, to cool these rooms the brine must be blended A separatethree-way blending valve and pump will be required for each room(see Figure 12.6)

Storage tank

Brine

chiller

Pump

Coil 1

Coil 2

Coil 3

Figure 12.5 Brine circuit for separate rooms

152 Refrigeration and Air-Conditioning

12.8 Corrosion

If brine circuits are open to the atmosphere, air may be entrained,with consequent oxidation, and the solution will become acid Thiswill promote corrosion and should be prevented as far as possible

by ensuring that return pipes discharge below the tank surface

To reduce the effects of corrosion, inhibitors are added, typicallysodium chromate in the salt brines and sodium phosphate in theglycols These are alkaline salts and help to counteract the effects

of oxidation, but periodic checks should be taken, and borax orsimilar alkali added if the pH value falls below 7.0 or 7.5 [1].Brines are hygroscopic and will weaken by absorbing atmosphericmoisture Checks should be made on the strength of the solutionand more salt or glycol added as necessary to keep the freezingpoint down to the required value

The preferred brine circuit is that shown in Figure 12.5, andhaving the feed and expansion tank out of the circuit, which isotherwise closed This avoids entrainment of air and too muchsurface exposure The same arrangement can be used with thedivided storage tank shown in Figure 12.6, except that the tank will

be enclosed, with a separate feed and expansion tank

12.9 Thermal storage by frozen brines and ice

Variations in cooling load can be provided from the latent heat ofmelting of ice or a frozen eutectic Ice can be formed by allowing it

Coil 1

Coil 2

Coil 3

Storage tank

Figure 12.6 Brine circuit for rooms at different temperatures

Liquid chillers Ice Brines Thermal storage 153

to build up on the outside of evaporator coils in a tank Brines aremore normally held in closed tanks or plates, again with evaporatorcoils inside, the outside of the tank forming the secondary heatexchange surface Eutectics can be formulated according to thetemperature required (see Figure 7.8)

A variation is to have a pumpable fluid such as one of the glycols,and to contain a eutectic solution within capsules in a storage tank.The capsules are in the form of plastic balls and the eutectic withinmay be formulated to suit any required thermal storage temperature.The capsules are frozen solid by pumping the glycol through anormal shell-and-tube cooler and then through the tank When thestored cooling effect is to be used, the glycol flow is diverted to theload, and the capsules then melt again This system has the advantage

of avoiding the corrosion effects of salt brine, and can be used atalmost any required storage temperature, depending on the eutectictemperature of the mixture within the capsules

A similar product is available for domestic use Plastic containershold a eutectic solution, and these are frozen down by placingthese elements in the domestic deep-freeze cabinet Once frozen,they can be used in picnic baskets, etc., for the short-term storage

of cold foods, wines, ice cream, etc

The use of ice cubes to cool beverages by contact or immersion

is well known

In commercial use, thermal storage has three main applications:

1 To handle a peak cooling load with a reduced size of refrigerationplant, typically to make ice over a period of several hours andthen use ice water for the cooling of a batch of warm milk on adairy farm This is also used at main creameries, to reduce peakelectricity loads The available water is very close to freezingpoint, which is the ideal temperature for milk cooling

2 To run the refrigeration system at night, or other times whenelectricity is cheaper, to avoid premium electricity rates, or toavoid maximum demand charges It is also in use in areas wherethe electricity supply is unreliable Where the cold water is to beused at a higher temperature, such as in air-conditioning, thecircuit will require three-way blending valves

3 As hold-over cooling plates in transport (see Figure 7.8d and

Chapter 20)

13 Packaged units

13.1 General

A high proportion of the total cost of a refrigeration or ditioning system is made up of work which can be carried out quicker,cheaper and under better managerial control within a factory ratherthan on the installation site This work includes the following:

air-con-1 Procurement, inspection and storage of bought-out items

2 Storage of manufacturing materials

3 Manufacture of in-house components

4 Assembly of parts into systems

5 Piping, wiring, charging, testing

A wide range of factory-built packaged units is now made, andcovers most requirements except the larger or more specializedinstallations

The advantages of packaged units are as follows:

1 Correct selection and balance of components

2 Assembly, leak testing, processing and charging under factoryconditions

3 Inspection and testing of the complete unit before it leaves thefactory

4 Delivery to the site complete and in working order, so avoidingsite delays for materials

5 Simplified site installation, with a minimum of disruption,inconvenience and cost

Disadvantages are that the unit may not be exactly the right size forthe duty, since a stock unit may be used, and the risk of misapplication

13.2 Condensing units

The basic condensing unit is a single package comprising the

Packaged units 155

compressor with its drive, the condenser (either air- or water-cooled)and all connecting piping, and the necessary controls to make theset functional (Figure 13.1)

Figure 13.1 Air-cooled condensing unit (Courtesy of Prestcold Ltd)

Such assemblies might have the compressor and drive only, forsite connection to a remote air-cooled condenser As such, they arecorrectly termed compressor units Compressor and condensingunits will be site connected to evaporators, and these componentsmust be matched in capacity (see Chapter 10)

Cooling capacity data will be based on various condensingconditions, in terms of air or water temperature onto the condenser,and for a range of evaporating conditions for which the set may besuitable [35]

Example 13.1 In the rating curves for an air-cooled condensingunit shown in Figure 13.3, what is the cooling capacity at anevaporating temperature of –25°C and with air onto the condenser

at 25°C? By how much does this drop with condenser air at 35°C?From curves rating at –25 to +25°C is 1310 W

rating at –25 to +35°C is 1085 W

Example 13.2 In the rating table shown in Table 13.1 for a cooled condensing unit, what is the cooling duty at –20°C evaporation,with water onto the condenser at 25°C?

water-156 Refrigeration and Air-Conditioning

Figure 13.2 Packaged water chillers (a) Air cooled (b) Water cooled(Courtesy of APV Baker Ltd)

From table, rating at – 20 to + 25°C is 18.6 kW

Since compressor and condensing units do not include an evaporator,they are not complete systems and will not be charged with refrigerant,but may have a holding charge of dry nitrogen, or a little of the

(b) (a)

Packaged units 157

refrigerant gas to maintain a slight positive pressure for transit.Suction and liquid interconnecting lines and wiring will have to beinstalled on site

13.3 One-piece packages

The true packaged unit will have all the parts of the system and will

be factory tested in the complete state There are four basic types:

158 Refrigeration and Air-Conditioning

Table 13.1 Capacity, in kW, of water-cooled

Air cooling, air cooled

Air cooling, water cooled

Liquid cooling, air cooled

Liquid cooling, water cooled

Ratings for such units will be published in terms of the enteringfluid on both the evaporator and condenser side (see also Chapter35)

The siting of a packaged unit is more critical than that of separateplant, since all components are together, and a compromise mayhave to be reached between the convenience of having the unitclose to the load and the difficulty of obtaining condenser air orwater, transmitting extra noise, or creating new safety aspects

13.4 Split packages

To avoid the constraint of having all parts in one package, theevaporator set may be split from the condenser, the compressorgoing with either (see Figure 13.4) The unit will be designed as acomplete system but the two parts are located separately andconnected on site On some small units, flexible refrigerant pipingmay be provided

If the system is of a range up to about 5 kW, coils of prechargedsoft copper tube, with self-sealing couplings, may be supplied forconnection within a limited distance of 5–15 m This facilityenables full factory processing to be carried out to the standards of

a one-piece unit It is limited to the availability of suitable tubing,usually

5

8 inch outside diameter In such systems, the total charge issuitable for the final assembly, and pipes should not be extendedbeyond the factory-supplied length without prior consultation withthe supplier

Larger split packages must be piped on site by normal methods,and then processed and charged as an open plant Split unitevaporators should not be located more than 5 m higher than their

condensers (see Section 11.8) See also multi-splits, Section 28.8.