Several decades ago the oil-cooling system using the external heat exchanger cooled by water or antifreeze lost favor compared to direct injection of refrigerant. While this concept in turn has been supplanted in many cases by the thermosyphon system, many new systems use direct refrigerant injection for oil cooling. Liquid refrigerant from the receiver is injected into the compressor at an early stage of compression in a manner illustrated by Figure 5.21. A valve regulates the flow of liquid to maintain the desired discharge temperature of the refrigerant/oil mixture. A conventional superheat-controlled expansion valve (sometimes called a TXV or a thermo-valve), as will be described in Chapter 11, Valves and Refrigerant Controls, can be adapted for this service. The usual valve controls the amount of superheat at the position of its sensor, but the objective of the valve in Figure 5.21 is to control the temperature at 49°C (120°F). The adaptation consists of providing a pressure reference to the valve, which it tries to match by the fluid pressure, and thus the desired temperature, in its sensing bulb.
Cooling the oil with direct injection of refrigerant imposes penalties both in the reduction in refrigeration capacity as well as an increase in power. Figure 5.22 shows the magnitude of these penalties for an ammonia compressor.
Not all manufacturers agree on the magnitude of power penalties, reporting instead that the power penalty is about 1% for all conditions. Both penalties increase with an increase in pressure ratio against which the compressor operates. The losses are thus significant when the compressor operates single stage with a low evaporating temperature. The losses are moderate for highand low-stage compressors in a two-stage system.
A limitation in the system operating with direct-injected compressors is how low the condensing temperature is permitted to fall. To conserve energy, most plants operate with as low a condensing temperature as the ambient temperatures can provide. One of the plant characteristics that may limit the degree to which the condensing pressure is allowed to fall is the need for adequate pressure to force liquid through the control valve in Fig. 5.21. A typical minimum condensing pressure recommended by manufacturers of screw compressors is 860 kPa (125 psia).
The decision of whether to choose direct injection as the method of oil cooling is primarily an economic one. A plant cooling oil with a thermosyphon system will be more efficient, but at the expense of a higher first cost in comparison to direct injection. Direct injection boasts the lowest first cost of the oil-cooling methods, and this feature is a major contributor to its selection.