Evaporators for liquid chilling

Liquid refrigerant inside the evaporator absorbs heat from the air being cooled or the water chilled and boils, to efect the refrigeration duty. Two types of evaporator are in use: a flooded version used mostly for water chilling and a dry expansion form used for both chilling water and cooling air.

C

 

Y

 

C

 

V

)

 

)

Refrigerant gas to compressor suction

 

Surge

Drum

 

Float valve

 

— Level of liquid refrigerant

Refrigerant bubbles rising to surge drum ty—CW feed

 

Evaporators for liquid chilling

Submerged evaporator coils containing liquid refrigerant

• Open-topped tank

 

Agitator ■

 

Evaporators for liquid chilling

Evaporators for liquid chilling

Refrigerant

Connections

Water overflows from the header tank and wets the outside of the evaporator coils

CW feed

Chilled water out

(yy) Circulating pump

(b)

Fig. 12.2 (a) Submerged evaporator, (b) Baudelot cooler.

(a)

подпись: (a)In a nominally closed circuit. If the water being chilled is in an open circuit (as with an air washer) the fouling factor adopted must be twice this, namely, 0.000 176 m2 K W-1 (0.001 ft2 h °F/Btu). The difference between the two fouling factors corresponds to between 5 per cent and 10 per cent of the overall thermal resistance, water-to-refrigerant and is significantly large. See also the discussion on fouling factors in section 12.14.

Shell-and-tube chillers are commercially available as part of air or water-cooled packages, complete with condensers and reciprocating or centrifugal compressors. It is important that the chilled water flow rate within the tubes is kept at a fairly constant value in order to avoid the risk of freezing the water, bursting the tubes and causing expensive damage to the refrigeration plant. It is also important that the plant is properly controlled: packages with reciprocating compressors must only be controlled from return water temperature and

Never from flow water temperature, because of the risk of freeze-up at partial load (see section 12.10). Moreover, for similar reasons, the minimum practical design chilled flow temperature is about 6.5°C for a plant with a reciprocating compressor, although as low as 5°C is possible with safety when using a centrifugal or screw machine with sufficiently refined capacity control.

The connection from the feed and expansion tank (or from the pressurisation unit) must be made at the suction side of the pump, which should blow through the evaporator because of its relatively high pressure drop. The whole of the chilled water system is then at a pressure above the datum imposed at pump suction by the feed-and-expansion connection, with less risk of cavitation, or dissolved gases coming out of solution within the pump impeller.

(ii) Submerged evaporators. If coils containing boiling refrigerant are submerged beneath the water or brine being chilled in an open tank the water can be reduced in temperature to as low as 0°C, with safety: as ice forms it pushes against the surface and does not crush the evaporator coils. An agitator is necessary to improve heat transfer (see Figure 12.2(a)) and baffles may also be fitted in the tank. Such evaporators are commonly used for special industrial, rather than commercial, applications. In these cases it is vital that the supplier of the refrigeration plant should establish at the design stage the fluid flow characteristics over the tubes: the presence of a glycol in the water, to permit sub-zero temperatures to be obtained, can change the value of the Reynolds Number enough to give transitional or even fully laminar flow, with a dramatic fall in the heat transfer coefficient on the water side.

(iii) Baudelot coolers. As Figure 12.2(b) shows, heat transfer is promoted by arranging for water to flow downwards over the outside of the evaporator tubes. Water can safely be produced at temperatures approaching 0°C. Baudelot coolers are not used in commercial applications.

For both the submerged evaporator and the Baudelot cooler the open chilled water vessel is its own feed-and-expansion tank and the large mass of chilled water present confers stability on the system performance by virtue of its thermal inertia. See section 12.10 for the relevance of storage in commercial applications.

(iv) Dry expansion evaporators. Water flows over the outside of the tubes, through the shell and past baffles. Liquid refrigerant is metered by a thermostatic expansion valve to produce about 8 degrees of superheat at the outlet to the suction line. The tubes can have extended surfaces and high rates of heat transfer are possible. Evaporators of this type are common in commercial liquid chilling packages with reciprocating compressors. The strictures on water flow, control and chilled water temperature, mentioned above, are especially important here. Dry expansion chillers are often used for cooling duties below about 140 kW and flooded evaporators are common above this. A liquid-suction heat exchanger is desirable for dry evaporators.

(v) Plate heat exchangers. These comprise parallel plates of stainless steel, separated by gaskets and bolted together. Liquid refrigerant and water to be chilled flow between alternate plates with a high rate of heat exchange. Some flexing in the steel sheets is possible and it is claimed that a measure of freezing can be tolerated, although this is highly undesirable because of the potentially dire consequences to the refrigeration system.

Posted in Air Conditioning Engineering


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