LIQUID COOLING EVAPORATORS

Liquid cooling evaporators may be direct expansion or flooded type. Flooded evaporators (Figure 7.2) have a body of fluid boiling in a random manner, the vapour leaving at the top. In the case of ammonia, any oil present will fall to the bottom and be drawn off from the drain pot or oil drain connection.

Refrigerant liquid level

Fluid inside tubes

LIQUID COOLING EVAPORATORS

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Refrigerant liquid level

(b)

(a)

LIQUID COOLING EVAPORATORS

Figure 7.2 F looded evaporators. (a) Shell-and-tube. (b) Jacketted. (c) Raceway

In the shell-and-tube type, the liquid is usually in the pipes and the 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 for the suction gas to escape clear of the surface without entraining liquid. Further features such as multiple outlet headers, suction trap domes and baffles will help to avoid liquid droplets enter­ing the main suction pipe. Gas velocities should not exceed 3 m/s and lower figures are used by some designers.

A sectional arrangement of a flooded shell and tube type is shown in Figure 7.3 . The speed of the liquid within the tubes should be about 1 m/s or more, to pro­mote internal turbulence for good heat transfer. End cover baffles will constrain the flow to a number of passes, as with the shell-and-tube condenser.

LIQUID COOLING EVAPORATORS

Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have flooded or direct expansion circuitry. Flooded coils will be connected to a combined liquid accumulator and suction separator (usually termed the surge drum), in the form of a horizontal or vertical drum (see Figures 7.2(c) and 7.4). The expansion valve maintains a liquid level in this drum and a natural circu­lation is set up by the bubbles escaping from the liquid refrigerant at the heat exchanger surface.

LIQUID COOLING EVAPORATORS

Shell and tube evaporators with direct expansion circuits have the refrig­erant within the tubes, in order to maintain a suitable continuous 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 (Figure 7.5), or may be shell-and-coil (see Figure 7.6). In both these configurations, baffles are needed

Water

Out

LIQUID COOLING EVAPORATORS

In

Figure 7.6 Shell-and-coil evaporator

LIQUID COOLING EVAPORATORS

Figure 7.5 Onda shell and tube direct expansion evaporators (Titan)

подпись: figure 7.5 onda shell and tube direct expansion evaporators (titan)

Water out

подпись: water out

T

подпись: tOn the water side to improve the turbulence, and the tubes may be finned on the outside. Internal swirl strips or wires will help to keep liquid refrigerant in contact with the tube wall.

The spray chiller operates with a much lower refrigerant charge than a con­ventional flooded evaporator does. In Figure 7.7 the liquid refrigerant level in the surge drum shell is kept below the tubes and liquid is pumped to spray noz­zles which ensure that the tube surfaces are covered with an evaporating liquid film. Water or brine passes through the tubes. The gas outlet to the compres­sor suction is in the upper part of the shell and a baffle arrangement prevents entrainment of liquid droplets. Due to the distribution of refrigerant very close control of the evaporation can be obtained. Evaporation ceases immediately when the liquid spray is stopped. For these reasons the brine can be cooled to a

LIQUID COOLING EVAPORATORS

Figure 7.7 Spray chiller complete with pump assembly. The pump is kept well below the liquid level to ensure adequate head (Titan)

Temperature close to its freezing point. Water can be chilled to a temperature of less than 1°C with an evaporating temperature close to — 2°C.

Direct expansion coils for immersion in an open tank will be in a continu­ous circuit or a number of parallel circuits (see Figure 7.8). Liquid velocity over such coils can be increased by tank baffles and there may be special purpose agitators, as in an ice-making tank. Coils within an open tank can be allowed to collect a layer of ice during off-load periods, thus providing thermal storage and giving a reserve of cooling capacity at peak load times (see also Chapter 12).

Refrigerant connections

LIQUID COOLING EVAPORATORS

(a)

Figure 7.8 Direct expansion tank evaporator. (a) Section

LIQUID COOLING EVAPORATORS

(b)

Figure 7.8 (Continued) (b) Elevation

Plate heat exchanger evaporators are now widely used. A heat exchanger of this type consists of a number of herringbone corrugated plates assembled to form a pack (Figure 7.9). The herringbone indentations are set in opposite direc­tions to each other in relation to each facing plate. Brazed plate heat exchangers (BPHX), Figure 7.10 have plates made from stainless steel with a copper coating on one side. During manufacture they are assembled and held together by the end plates, and heated under vacuum conditions. The copper melts and coagulates at the contact points and seals the edge joints. When cooled a structure of alternate counter flow channels is formed, separated only by a thin layer of stainless steel.

LIQUID COOLING EVAPORATORS

Figure 7.9 Plate heat exchanger flow diagram (Alfa Laval)

LIQUID COOLING EVAPORATORS

Figure 7.10 Brazed plate heat exchanger assembly (Alfa Laval)

The volume of refrigerant contained in a heat exchanger of this type is approximately 2 litres for each square metre of cooling area, which is up to 10 times lower than for multi-tube designs. This helps to keep refrigerant charge level low and offers a rapid response to changes in energy demand. The turbu­lence induced by the pattern of the channels results in very high heat transfer coefficients, typically three to four times greater than with conventional tubu­lar designs. The counter flow gives temperature differences close to the ideal. A refrigeration BPHX always has all the refrigerant channels surrounded by water channels so that there is one more water channel than the number of refrigerant channels. The outermost ones are water channels.

Many configurations are possible. If the channel height is decreased or the corrugation angle is increased the pressure drop and heat transfer rises. Increasing the length of the plates has a similar effect.

When used as a direct expansion evaporator the refrigerant velocity should be high enough to entrain oil that remains after evaporation is complete. Where conditions give rise to non-miscibility, the formation of oil film on the wetted surface can impair heat transfer. On the superheating section there is less effect because this region is sensible heat transfer and velocity to carry the oil drop­lets upward is the requirement.

It is important to ensure good distribution so that the refrigerant enters all the channels evenly. The BPHX should be mounted vertically with the refriger­ant entry at the bottom. The pipe between the expansion valve and the entry point should be short and of small diameter so that the liquid velocity carries the droplets through to the far plate. Some designs incorporate distributors to aid this process. Mal-distribution can cause erratic expansion valve behaviour and depress the evaporation pressure. Electronic expansion valves, which pro­vide continuous flow, such as variable orifice types, are suitable, but due to the small internal volume of a BPHX, a pulse-modulated valve may give rise to unacceptable pressure fluctuations.

Refrigerant

LIQUID COOLING EVAPORATORS

Figure 7.11 Witt Plate and shell heat exchanger (Titan)

Larger installations can use plate and shell heat exchangers (Figure 7.11) , which work on a similar principle. In this case a welded construction is used making this type of evaporator suitable for ammonia and carbon dioxide refrigerants. They can be kept flooded with refrigerant, working in conjunction with a surge drum into which the liquid is metered by a float expansion valve. An example of this assembly is shown in Figure 7.12 .

To compressor From expansion

LIQUID COOLING EVAPORATORS

Oil drain

Figure 7.12 Witt Plate heat exchanger and surge drum assembly (Titan)

Where water is to be cooled close to its freezing point without risk of dam­age to the evaporator, the latter is commonly arranged above the water-collection tank and a thin film of water runs over the tubes. Heat transfer is very high with a thin moving film of liquid 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 7.13). It may be open, enclosed in dust-tight shields to avoid contamination of the product (as in surface milk and cream coolers), or may be enclosed in a pressure vessel as in the Mojonnier cooler for soft drinks, which pressurizes with carbon dioxide at the same time.

Chilled water

Figure 7.13 Baudelot cooler

подпись: 
chilled water
figure 7.13 baudelot cooler
Water distribution troughs

Vertical evaporator plates

Water tank

Circulating pump

Some liquids, such as vegetable fats and ice-cream mixes, increase consid­erably in viscosity as they are cooled, sticking to the heat exchanger surface. Evaporators for this duty are arranged in the form of a hollow drum (see Figure 7.2(b)) 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 and impelling the cold paste towards the outlet.

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