The reciprocating compressor
It is customary when considering the performance of reciprocating compressors, to do so for a particular condensing temperature because it is the performance of the condensing set that is really being considered. As the condensing pressure against which the compressor has to deliver the gaseous refrigerant rises, more work is expended per kW of refrigeration, and the refrigeration capacity of the condensing set reduces. On the other hand, as suction pressure at inlet to the cylinders of the compressor rises, less work need be done to secure the same refrigerating effect. Furthermore, with an increase in suction pressure, the density of the gas entering the cylinders also increases and so, for a given swept volume, the mass of refrigerant handled by the compressor becomes greater. A picture is thus formed of the characteristic behaviour of a condensing set. Figure 12.5 illustrates this.
Saturated suction pressure Fig. 12.5 Condensing set performance.
A compressor has a rising capacity characteristic; the higher the suction pressure the greater the capacity. The characteristic lines for different condensing temperatures are virtually straight, particularly if suction pressure is used as the abscissa. If the saturation temperature corresponding to suction pressure is used, superheat being ignored, some slight curvature results. Although straight lines can be used, the inaccuracy resulting being accepted, it is better to draw the lines as curves if the data are available for this purpose. They usually are, since compressor characteristics cannot be readily guessed. Reference must be made to the manufacturers for the information needed to plot the characteristic.
The capacity of a compressor, for a given condensing temperature, depends directly on the mass of refrigerant being pumped. Thus, if the speed of rotation or the number of cylinders used is altered, the refrigeration capacity of the compressor will change in direct proportion. This fact enables a family of characteristics to be plotted, provided, of course, that the initial information is to hand from the manufacturers.
Although the picture may eventually change, the majority of refrigeration compressors in use today are of the reciprocating type. These have from 1 to 16 cylinders, arranged in line, in V or W configuration, or sometimes in a radial disposition. The V or W arrangements are commonest. Most machines are driven by electric motors and are close-coupled or direct-coupled, although vee-belt drives are also used. Engines of various sorts have also been employed to drive compressors.
In common with most other types of compressor, reciprocating machines are available in open, hermetic or semi-hermetric form. The compressor of an open machine is driven by a separate motor, through a direct-coupling or a vee-belt. A seal is provided where the compressor shaft emerges from the casing but there is always the risk of some leakage of
refrigerant gas out of the system. The driving motor is cooled in a conventional manner. With the hermetic machine the compressor and the driving motor share the same shaft (they are close-coupled) and both are contained in a welded casing. Comparatively cold, suction gas flows over the stator winding of the motor before entering the compressor. The advantages of the hermetic arrangement are that leakage at a shaft seal is eliminated, the shaft is shorter and more rigid, the bearing arrangement is simplified, the machine is quieter and the motor is gas-cooled. A potential disadvantage is that, if the control of the related air conditioning system is poorly designed or ineptly operated, or if the compressor is allowed to start too frequently, there is a greater risk of motor burn-out than with open machines. If a burn-out occurs, the consequent reaction with the refrigerant poisons the rest of the refrigeration system, necessitating thorough cleaning, or even replacement. Semi-hermetic machines are similar but have a removable cover bolted on to the end of the casing to facilitate occasional maintenance. Welded, hermetic machines are used for the smaller duties, up to about 70 kW of refrigeration and bolted, semi-hermetics are used for larger loads. Packaged, water-chilling units having multiple, semi-hermetic compressors are used with capacities up to about 700 kW of refrigeration.
The principal method adopted for regulating the capacity of a reciprocating compressor is cylinder unloading. Cylinders are unloaded by arranging to hold the suction valve open, the discharge valve staying closed because of the high pressure on its other side. As the piston moves up and down, gas is circulated through the suction valve, no compression is achieved and the cylinder makes no contribution to the refrigeration duty. The suction valve is held open mechanically, using the pressure developed by the oil lubrication system. A fall in the suction pressure is usually the signal that initiates cylinder unloading. A suitable differential gap separates the measured values of suction pressure at which loading and unloading occur. A less common alternative is to measure some external, physical property, such as the temperature of the water returning to a chiller. A fall in this is then the indication for unloading. An electrical signal from the temperature sensor may be used to open a solenoid valve in a pipeline feeding lubrication oil to the cylinder unloading mechanism. The use of suction pressure is neater because all the unloading mechanism is within the compressor whereas, with the other method, external connections must be made. Figure
12.5 illustrates what happens when a pair of cylinders in a four-cylinder machine is unloaded.
Proper lubrication is essential and must be ensured under all operational conditions. Halogen type refrigerants are miscible in oil and tend to collect in the crankcase of reciprocating machines. When the compressor starts, the low pressure developed in the crankcase causes the refrigerant to boil and the mixture foams. Foaming is bad for the lubrication of the system and should be prevented by the provision of a crankcase heater. This is electrically energised when the machine is off and is de-energised when the compressor runs. The crankcase heater is wrapped externally around the casing of hermetic machines but is fitted within the crankcase of semi-hermetic and open compressors. Other refrigerants that do not use mineral oil as a lubricant (see section 9.11) have their own characteristics, which must be established.
The inefficiency of the motor reduces the coefficient of performance of hermetic and semi-hermetic machines, in comparison with open-drive compressors. This is shown by equations (12.1) and (12.2):
^ N Refrigeration capacity (kW)
COP (hermetic) = —=————————- —rrrr-
Input power to motor kW
X Refrigeration capacity (kW)
COP (open) = — r—f————— r. — (12.2)
^ Input power to shaft (kW)
Posted in Engineering Fifth Edition