The thermosyphon concept in oil cooling achieves heat transfer by boiling liquid refrigerant at the condensing pressure. Furthermore, the boiling refrigerant flows by natural convection (thermosyphon effect) through the heat exchanger. Flow diagrams of two thermosyphon systems are illustrated in Figures 5.23 and 5.24. Liquid refrigerant flows down from a receiver and some of this liquid boils in the heat exchanger. The mixture of liquid and vapor rises from the heat exchanger and returns to the receiver. Circulation of the refrigerant in the loop takes place because of the greater desity in the liquid leg in contrast to the return line where the mixture of liquid and vapor flows.
The difference of the systems in the two diagrams is that in Figure 5.23 that the liquid level in the system receiver is above that of the heat exchanger, a requirement for the natural circulation to take place. If the level in the system receiver is at or below that of the heat exchanger, as in Figure 5.24, an additional receiver, called the thermosyphon or pilot receiver, must be installed.
In the receivers of both thermosyphon systems shown in Figs. 5.23 and 5.24, the entrance to the liquid line that drains refrigerant to the oil cooler is positioned lower than the entrance to the line feeding liquid to the evaporators. The strategy is that the oil cooler has the first call on liquid refrigerant. If the high side of the system should run short of refrigerant, the liquid supply for cooling the oil has first priority in order to protect the compressors.
The designer of the refrigeration system is normally not called on to design the heat exchanger that cools oil. In fact, manufacturers usually guard jealously their right to select the heat exchanger. In the first place, the heat exchanger is an integral part of the package that is assembled at the factory. Of equal importance is that the heat exchanger is a minor contributor to the total cost of the package, and an improper heat exchanger could result in damage to the expensive compressor.
Each compressor is equipped with its own heat exchanger, and in multiplecompressor installations one thermosyphon system can supply several heat exchangers The designer of the refrigeration system is responsible for designing the thermosyphon system which includes selecting the size of the thermosyphon receiver and the sizes of three main lines. These pipes include: (1) the liquid/vapor line from the heat exchanger to the receiver, (2) the liquid line from the receiver to the heat exchanger, and (3) the vapor line from the receiver to the header carrying discharge vapor to the condenser(s). The diagrams of the thermosyphon systems shown in Figures 5.23 and 5.24 are schematic, and there are additional piping details that must be provided for proper drainage of the condenser, as will be discussed in Chapter 7 on condensers.
A basic piece of data is the fraction of the heat equivalent of the total of the compressor power and refrigeration load that is absorbed by the injected oil and thus must be removed in the oil cooler. This percentage is presented in Figure 5.25 and is needed for the thermal analysis of a two-stage system. It would also be used if the designer is making detailed calculation of the thermosyphon system. To simplify the task of designing the thermosyphon system, Reference 11 bypasses some of these calculation details by providing recommendations on the size of these components, and those guidelines will be quoted here.
The preliminary steps in the basic procedure of selecting the components in the system are to determine the flow rates.
Thermosyphon receiver. The maximum heat rejection rate of the heat exchanger controls the size of the receiver. Specifically, the size of the receiver is chosen so that a reserve for five minutes of operation, thus , is available if the supply of liquid from the condenser is interrupted. It is expected that the outlet to the system receiver is at about the midpoint in the thermosiphon receiver. Thus, the thermosiphon receiver should be twice the size of the volume of five minutes of refrigerant evaporation.
Figure 5.26 shows recommended volumes for R-22 and ammonia on this basis. Usually these receivers are about 1.7 m long (5 or 6 feet). Figure 5.26 shows that a smaller receiver is adequate for ammonia compared to R-22, because the high latent heat of ammonia permits it to transfer a given amount of heat with a lower flow rate than for R-22.
Liquid line from receiver to the heat exchanger. This section of line carries a flow rate greater than the rate evaporated, because a properly operating thermosiphon system circulates unevaporated liquid back to the receiver. Designers of thermosyphon systems strive for a circulation ratio of 3:1 for ammonia and 2:1 for R-22, where the circulation ratio means the rate supplied to the heat exchanger divided by the rate evaporated.
The recommended pressure gradients11 for this pipe are 22.6 Pa/m (0.1 psi per 100 ft) for ammonia and 113 Pa/m (0.5 psi per 100 ft) for R-22. With the assumption of a condensing temperature of 35°C (95°F) at which temperature the enthalpy of evaporation for ammonia is 1124 kJ/kg (483.2 Btu/lb) and for R-22 it is 172.6 kJ/kg (74.2 Btu/lb), the following equations may be used to compute the required pipe size, D, in inches to abide by the pressure gradients and circulation ratios specified above.
Liquid/vapor line from heat exchanger to thermosiphon receiver. The recommended pressure gradients11 for the liquid/vapor return line are 9.04 Pa/m (0.04 psi per 100 ft) for ammonia and 45.2 Pa/m (0.2 psi per 100 ft) for R-22.
To abide by these pressure gradients, the required pipe sizes are given by the following equations:
Vapor line from the receiver to the condenser header. This line may at first have the appearance of an equalizer line through which there is flow only when the pressures at the terminal points of the pipe need to be balanced. If that were the case, only a small pipe, perhaps of 1-inch size, would suffice. On the contrary, a flow of refrigerant equal to passes through this line. To motivate this flow, the pressure in the thermosiphon receiver must be higher than the entrance to the condenser. In other words, the refrigerant must gain in pressure as it passes through the condenser and the drain line to the thermosyphon receiver. A way in which this can be done is to trap the liquid drain line from the condenser and provide a liquid leg in this line to compensate for the pressure drop. The details of this arrangement will be provided in Chapter 7, but for now it is important to realize that the size of this line should be generous to keep the pressure drop low.
The recommended minimum pipe sizes11 for various flow rates with ammonia and R-22 are given in Table 5.3.
Difference in elevation from the receiver to the heat exchanger. The thermosyphon concept operates because of the higher pressure developed down the liquid leg in comparison to the magnitude of pressure reduction of the less-dense mixture of liquid and vapor flowing upward in the line between the heat exchanger and the receiver. Since the pressure difference is proportional to the vertical distance over which this difference in density prevails, a certain minimum vertical distance should be provided between the liquid level in the thermosiphon receiver and the heat exchanger. Reference 11 recommends a minimum elevation difference of 1.8 m (6 ft).