EVAPORATORS |
Natural Convection
Plate type | Bare (plain tube)
| Dry Expansion | Flooded |
Evaporators for chilling Liquid
| Double Pipe (coaxial) | Shell & Coil | Shell & Tube | Baudelot Coolers | Tank Coolers | Further Reading
The evaporator is a heat transfer system, and is that part of a refrigeration cycle in which liquid refrigerant is evaporated for the purpose of removing heat from the refrigerated space or product.
Evaporators are manufactured in a wide variety of types, shapes, sizes and designs, and are classified in the following way:-
The capacity of any evaporator or cooling coil is the rate at which heat will pass through the evaporator walls from the refrigerated space to the vaporising liquid.
PLATE TYPE EVAPORATOR |
The refrigerant is contained between two plates to give a large cooling surface (aluminium corrugated and prefromed in a ‘U’ shape). This type of evaporator is widely used in domestic refrigeration. The only maintenance requirement is to keep the plate free from ice (See Figure 1)
Figure 1
BARE (PLAIN TUBE) TYPE |
This type consists of a bank of tubes, copper or steel. They are used for cold rooms with natural air circulation over the tubes. Maintenance consists of keeping the internal walls of the tubes free from oil and external surfaces clear of ice.
On some commercial plants, pipes containing refrigerant are sealed between two dole-plates. The gap between the plates is filled with a low temperature freezing point (eutectic) solution to improve heat transfer. Alternatively, the gap is evacuated to form a vacuum. This vacuum type relies on heat transfer being conducted through metal contact only. (See Figure 2)
Figure 2
| FORCED CONVECTION EVAPORATORS |
High velocity air over a closely finned coil can increase heat transfer. The use of forced draught can reduce the overall size and weight of the evaporator which can reduce purchasing and installation costs. Forced draught also gives:-
The fins on a forced draught evaporator are very closely spaced which gives a high capacity rating. Defrosting must be carried out more frequently. It is possible to operate these coils at about 60% flooded conditions (HighVelocity air is forced through the finned coils with powerful fans. The fan motors can be single or three phase).
| DRY EXPANSION EVAPORATORS |
Dry expansion evaporators are so called because liquid refrigerant enters the evaporator by an expansion valve and vapourises by the time it reaches the end of the evaporator coil. There is always at least 20% vapour present within the evaporator pipework. The amount of liquid present in dry expansion evaporators will depend upon the fixture load. At light load conditions, the amount of liquid will be small, and in high load conditions, the amount of liquid will be large. The larger the wetted surface, the greater the efficiency. (See Figure 3)
Figure 3
| FLOODED EVAPORATORS |
A flooded evaporator is completely filled with liquid refrigerant, and the level of the refrigerant is maintained in an accumulator by a float-valve. Evaporators are connected in parallel to the accumulator. One pipe from the bottom of the vessel supplies liquid refrigerant. The return pipe containing liquid refrigerant and vapour enters the vessel near the top. The liquid from the accumulator is quite often pumped around the evaporators.
The disadvantage of the flooded evaporator is that it requires a considerable charge of refrigerant and is bulky. The main advantage is that the inside of the pipes are completely wetted with refrigerant and this produces a very high rate of heat transfer. (See diagram in Dossat Page 230)
EVAPORATORS USED FOR CHILLING LIQUID |
Evaporators used to cool liquids can be divided into five general types:-
DOUBLE PIPE COOLERS |
This cooler consists of one pipe inside another. The fluid to be cooled flows in the inside and the refrigerant flows in the outer space between the inner and outer tube in the opposite direction to the fluid to obtain a high heat transfer rate – counter-flow action (See Figure 4)
Figure 4
SHELL AND COIL EVAPORATOR |
Shell and Coil evaporators have one or more spiral shaped, bare tube coils enclosed in a welded steel shell. They are generally dry expansion type with refrigerant in coils and chilled liquid in the shell. They have thermal capacity for application for high but infrequent loads. They are used for chilling of drinking water and other aspects where cleanliness is a factor, e.g. Bakeries and Photographic Laboratories. (See Dossat, Page 227)
SHELL AND TUBE EVAPORATORS |
Shell and Tube evaporators consist of a cylindrical shell in which there are a number of straight tubes arranged in parallel, held in place by end plates. When in operation, refrigerant is expanded in the tubes with chilled liquid in the shell. Shell diameters range from 150 mm to 1.5 m, numbers of tubes from less than fifty to several thousand. Tube diameters range from 16 mm to 50 mm with tube lengths 1.5 m to 6 m. Steel tubes are used for ammonia and copper tubes used for other refrigerants. Copper tubes give high heat transfer coefficients. Evaporators using Freon refrigerants often have fins on the refrigerant side, i.e. internally (because of low film conductance of these refrigerants).
The shell and tube evaporators are very common, highly efficient, compact and readily available. They are used for both Dry and Flooded applications.
BAUDELOT COOLERS |
The Baudelot Cooler consists of a series of horizontal pipes, one under another, connected to form a circuit. They can be either dry or flooded. Refrigerant flows through the tubes, while chilled liquid flows in a thin film over the outside, (in a counter flow direction). This is then collected in a trough at the bottom of the cooler. They were used for cooling milk (prior to the introduction of the E.E.C. Regulations) wine and water, since it is possible to chill to very nearly freezing point without damaging equipment. (See Dossat, Page 225)
TANK TYPE COOLERS |
The Tank type cooler consists of bare tube refrigerant coils installed in the centre or at one end of a large tank containing chilled liquid. The tubes are submerged in the liquid but separated from the main body by a baffle. An agitator circulates liquid over the coils at a velocity of 0.5 m/s. Coil forms are spiral or race-way. The coils are operated flooded. Applications involve chilling water, brine or other liquids used as a secondary refrigerant. They are particularly suitable for applications having frequent and severe fluctuations in load. Large storage tanks are used to minimise the rise in temperature of chilled liquid during peak demand. (See Figure 5)
Figure 5
To summarise some of the points about coolers, we can classify them as follows:-
Sub-classified as to whether refrigerant, either (a) inside tubes or (b) outside tubes with a corresponding classification of dry or flooded.
Has a thermostatic expansion valve metering just the correct amount of liquid refrigerant to maintain predetermined degree of superheat
Has a float valve maintaining liquidate constant level, ie, as fast as refrigerant evaporates, more liquid is admitted by the float.
Recommended reading |
Principles of Refrigeration – R.J.Dossat (3rd Edition)
Chapter 11 Paragraphs 11. 1, to 11. 30 (Pages 199 – 23 1)