SECONDARY REFRIGERANT SYSTEMS.

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Secondary refrigerants | Benefits available from the use of secondary refrigerants | Disadvantages of Secondary Refrigerants | Types and Properties of Secondary Refrigerants | Calcium Chloride Brine | Factors to be considered in selecting a brine | Conversion of Hydrometer readings

Secondary Refrigerant Systems

Secondary refrigerants

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Secondary refrigerants are usually liquids, and are used to transfer heat from the substance being cooled to a heat exchanger where the heat is absorbed by a primary refrigerant. In an air conditioning system it could be said that air is acting as a secondary refrigerant.

These fluids are cooled by a primary refrigerant then exposed to the source, being sensibly heated by that source, thus absorbing its energy, and the warmer fluid returned to the chiller, rejecting the heat to the primary refrigerant.

High temperature applications such as air conditioning use chilled water as the secondary refrigerant while low temperature applications use brines, glycols and oils.

Large refrigeration plants often use secondary refrigerants to transport the cooling capacity from the plant room to the point of use.

A Basic Secondary refrigeration System.

Benefits available from the use of secondary refrigerants:-

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Disadvantages of Secondary Refrigerants:-

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Types and Properties of Secondary Refrigerants

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A good secondary refrigerant should have the following features:

The simplest secondary refrigerant is water, much used in air conditioning work above 0 C, the corrosion problem is minimal with a closed system. Used where application of control and flexibility allows some expenditure above the minimum.

Obviously, water has its limitations for lower temperature work due to its freezing, but this disadvantage can be overcome by adding a salt to form a brine. In general brines can be divided into four classes:

  1. Brines with a salt base, commonly being water solutions of calcium or sodium chloride.
  2. Brines with a glycol base, commonly water solutions of ethylene or propylene glycol.
  3. Brines for low temperature heat transfer being pure substances, such as methylene chloride, trichloroethylene, Rll, acetone, methyl or ethyl alcohol.
  4. Brines for special or unusual applications due to availability, extreme temperature range, experimental usage, etc. Typical examples are a petroleum product, synthetic oil, a silicone fluid, or sugar brine.

1 and 2 act on the phenomenon that water with a salt or other soluble material added has a depressed freezing point. The freezing point will obviously depend on the amount of salt in the water (i.e. proportion or concentration of the salt or material in the water. Each mixture has a concentration at which the freezing is a minimum. This concentration is the Eutectic Concentration. The minimum freezing point is the Eutectic Temperature and the point in the Temp/Conc. diagram is the Eutectic point. Two physical change lines exist on either side of the Eutectic Point namely the freezing line and crystallising or solubility line. The freezing line is found by taking a liquid or concentration below the eutectic Concentration, cooling it and in the form of ice, crystals will appear at a certain temperature leaving a liquid of higher concentration. Continued cooling will result in more crystals, further increase of concentration and so on. The freezing line is obtained by joining the respective freezing points.

The solubility line is found by taking a liquid sample whose concentration is above the eutectic concentration and cooling it. In this situation salt crystals will form at a certain temperature, reducing the concentration.

Further cooling causes more crystals to form and so on. The crystallising line is obtained by joining the respective points of crystallising. In practice the line around the Eutectic Points are vague and rarely published.

Eutectic points are:

Calcium Chloride Brine

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Calcium chloride brine is the commonest secondary refrigerant used in industrial and marine refrigeration systems.

Water is used for air conditioning installations (with suitable safeguards to prevent its freezing).

The density of the brine solution used has to be increased as the minimum temperature to be used decreases. The freezing point varies as in the table below.

CALCIUM CHLORIDE / BRINE

Specific Gravity

Hydrometer Reading

(Twaddell)

Freezing Point of Solution

C

F

120

40

-21

-6

121

42

-23

-9 5

122

44

-25

-13

123

46

-27

-17

124

48

-30

-21.5

125

50

-32

-26

126

52

-35

-31

127

54

-38

-37

128

50

-42

-44

129

58

-51

-60

In using the Twaddell hydrometer, the brine sample should be at +15C (60F) as the instrument is calibrated for use at this temperature.

In the absence of air, calcium chloride is not severely corrosive (steel brine pipes remain in good condition internally, but steel brine header and makeup tanks suffer severe corrosion at the brine/air interface). However, it is desirable to keep the brine slightly alkaline, with pH between 80 and 85. If found to be acid (e.g. litmus test papers), caustic soda should be added.

Brine heaters contain steam heated coils within a shell through which brine is circulated. When warm brine is required for defrosting, the steam should be put on first and the brine now restricted by first cracking the inlet brine valve until the brine has risen in temperature to above OC, (32F). An initial high flow rate of cold brine can cause freezing of the steam condensate with subsequent blockage. A brine temperature of 43C (110F) is suitable for defrosting.

Factors to be considered in selecting a brine

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  1. Cost —generally aqueous solutions are cheaper than the pure liquids even allowing for mixing inhibitors and water treatment.
  2. Corrosion —primarily, with salt based brines corrosion is controlled by density regulation, with high density fluid having less air trapped between the more closely packed molecules. (Limited by extra pumping power required and with a high alkalinity. It is recommended that pH values of 7.5 to 8.5 should be considered, no galvanising present 9 — 9.5.
  3. Corrosion inhibitors in the form of Chromic Acid or Sodium Dichromate used to be added but due to the heavy metal content, disposal, handling and control of these substances makes them prohibitively difficult to use. Unfortunately, there is no simple method for determination of sodium dichromate concentration. and a sample of brine should be sent for analysis at least every six months when this treatment is being used.
  4. The more modern tendency is use only suitably corrosion resistant materials such as ABS plastic, copper, cupro-nickle and cast iron (grades of stainless steel lower than 316 are not suitable as the oxide layer is not adequate to stop the chloride leaching the iron out of the alloy and causing pin hole perforations).
  5. The glycol solutions are generally less corrosive, but may require inhibitors for specific applications.
  6. The pure ‘brines’ are not corrosive provided they are not contaminated with impurities such as moisture. Also, methylene chloride and trichloroethylene must not be used with aluminium or zinc and they will attack most rubber compounds and plastic.
  7. Toxicity — is important if there is exposure to food or operating personnel. Sodium chloride and propylene glycol have low toxicity and inhibited propylene glycol is being increasingly used in food plants. All other brines are toxic to some extent or produce odours, requiring closed circuits.
  8. Flash Points — of acetone, methyl alcohol and ethyl alcohol require precautions against fire and explosion.
  9. Specific Heat — of the brine determines the required mass flow rate at a given temperature rise for the cooling load in question. The low temperature brines have a specific heat value, about one—third to one quarter of the water soluble brines.
  10. Density — is not a significant factor other than from corrosion control and possible pumping costs.
  11. Stability — is necessary at high temperatures where the brine may be heated. Methylene trichloride may break down into acidic products above 140 F.
  12. Viscosity — of brines affects pumping costs and the heat transfer coefficients. Obviously the viscosity of water based brines increases as they approach freezing. The pure substances have the property of low viscosity nearly down to freezing.
  13. Freezing Point — is the low limit that the brines can be used. In practice to avoid the risk of freeze up operating temperatures should be maintained 5 to 7 C (10 to 15 F) above the freezing point.
  14. Vapour Pressure — is important for brines used in an open system, especially if the brine warms to room temperature between usage. Possible risks are vapour losses, toxicity and inflammability.
  15. Water Solubility — of a brine in an open or semi—open system can be important due to the risk of dilution of a salt based or glycol based brine, or the formation of water ice in a pure brine which will be pumped around the system and deposited on heat exchange surfaces.
  16. Foaming — occurs if carbon dioxide in one of its phases has been immersed and bubbles through the brine for cooling OR an agitation may introduce air into the brine with risk of corrosion. An anti—foaming substance such as octyl—alcohol or triamylamine may be added to the brine.

Conversion of Hydrometer readings

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Comparative hydrometer readings

Specific

gravity

Twaddell

[ Tw ]

Beaum

[ B ]

on

Salinometer

Sp. Gr.
on
hydrometer

1000

0

00

0

1,000

1074

148

100

40

1,074

1100

20

132

53

1,100

1115

23

150

60

1,115

1125

25

161

64

1,125

1150

30

189

76

1,150

1160

32

200

80

1,160

1175

35

216

86

1,175

1200

40

242

97

1,200

1208

416

250

100

1,208

1225

45

266

107

1,225

1250

50

290

116

1,250

1261

522

300

120

1,261

1275

55

313

125

1,275

1300

60

335

134

1,300

 

Secondary Refrigerant Systems

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The evaporators (brine coolers) pumps and distribution valves in industrial installations and on cargo ships are usually located together within an insulated brine room, to provide ease of access and to eliminate the need for insulating individual items and pipes. Entering a brine room for the first time can be bewildering and consulting the brine diagram of a complex installation may at first increase the bewilderment. However, the system is basically very simple, the complexity arises from duplication of components and alternative cross connections.

The following diagrams attempts to illustrate the development of a brine system.

 Part (a) above, is the basic diagram with one chamber battery circulated with brine from one evaporator, in a closed circuit with a header tank to allow for expansion and contraction of the brine.

 Part (b) above, shows the addition of headers which enable a number of spaces to be served.

 (c) above, shows the addition of a second evaporator, air vent pipes have also been added at this stage.

 

Part (d) above, shows the addition of a third pair of headers served by a brine heater and third pump, so that any battery can be individually defrosted by circulating the warm brine. Also introduced is a brine "injection cross connection from the delivery of pump No. 1 to the suction of' pump No. 2. Brine injection is used so that evaporator No. 1 can assist evaporator No. 2 when No. 1 is set to deliver brine at a lower temperature than No. 2. A further refinement of this injection is the by-pass arranged across the inlet and outlet of evaporator No. 2 so that the cooling, of the brine circulating in No. 2 system can be achieved entirely by injection if desired.

In practice, there also has to be a brine make-up tank. In which solid calcium chloride is dissolved, for topping up the system. An overflow connection from the header tank, a safety pressure relief line from the brine heater, and a sighting connection to which the return from any space can be diverted, are all arranged to terminate over this make tip tank. These have been omitted from the above circuits for clarity.

 

 A typical secondary refrigerant system using ammonia as the primary refrigerant in the three independent chillers and Calcium chloride brine as the secondary refrigerant.

 

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