Why Control The Performance Of The Compressor?
Refrigeration and air conditioning loads are rarely permanent. The compressor is often asked to do only part of the work for which it is designed. When the system is operating at partial load, the suction pressure and temperature are lower than they are under a fully loaded conditions. Freezing of moisture that collects on the evaporator can result. "Frost" to the full extent reduces the amount of air that can pass through the coil, which in turn reduces the pressure and suction gas temperatures even further. Once frost begins a "snowballing" effect, which is rapidly degraded in absolutely locked coil. The airflow is blocked, and the compressor is shut down by security measures. Compressor damage may result if the condition persists.
Riding With The Load
To a certain extent, the compressor automatically regulates power down as the system load is reduced. This is true even compressor has no means to mechanically control of its capacity.
This natural potential to adjust the power load is expressed by the people in our business, when they say, "compressor goes with the load or the compressor swims with the load." About it speak and all mechanical refrigeration system.
To understand how this happens, you should understand the ideas of a certain volume, volumetric flow, mass flow, as they are used for the transfer of the gaseous refrigerant. Compressor operating on a permanent ABOUT the moves standing volume per unit of time. For example, piston compressor, pumping 10 cubic feet of gas refrigerant per minute "Forcing" a constant volume of 10 cubic feet. Its volume flow, therefore, is 10 cfm. As long as the compressor operates without compromising speed or handling cylinders, it will continue to replace gas at this rate.
Its capacity for heat transfer, however, is determined by its mass flow, not the rate of flow. That is, its ability to move heat depends on how many pounds of refrigerant (mass) of the pumps in a unit of time (Ib/min), not how many cubic feet of refrigerant it moves in a unit of time (cfm). Pounds, he pumps will be changed whenever the pressure on the absorption of changes. This means that while the volumetric flow rate is constant, the mass flow rate and power will change when the system operating conditions.
The specific volume of the substance, the volume, the space occupied by 1 pound mass of the substance. This is the reciprocal of density. That is the specific volume = 1/density.
As the pressure of the gas increases, 1 pound it can be made to fit into a smaller and smaller area or volume. In other words, the specific amount decreases with increasing pressure. Similarly, when the pressure decreases, 1 pound any gas will expand to fill more and more of the size or the volume of space. The unit volume increases, and the pressure drops.
Numbers on the right side near the saturated liquid curve in this P-H chart for R-22, show that the specific volume of the saturated vapour refrigerant decreases as the pressure increases and increases and the pressure goes down. 82 PSIA (40F saturation temperature), R-22 saturated vapor (point 1) has a specific volume of about 0.7 CFM/lb. If the pressure increases to 100 PSI (50F saturation temperature), we see point 2, that a given amount of refrigerant drops to a little less than 0.6 cu.ft./lb. If the pressure falls to 70 PSI (30F saturation temperature), we see that in point 3 the unit volume increases by approximately 0.8 cu.ft./lb.
Now, when we understand how pressure affects the specific volume, we can show how compressor "rides" with a load on the system. Let's assume that our 10 cfm compressor operates at 40F saturated suction temperature. The specific volume of gas refrigerant entering about 0.7 CFM/lb. This means that the compressor will be pumped about 14.3 pounds of refrigerant per minute:
10 cfm + 0.7 cu.ft./lb. = 14.3 pounds per minute
Now cooling load drops. Less heat absorbed by the evaporator, so less gas is brewed from liquid to vapor. Suction pressure falls and the saturation temperature is 30F. Under this pressure, the specific volume of gas approximately 0.8 cubicft./lb. The compressor will now pump only about 12.5 pounds of refrigerant per minute:
10 cfm 0.8 cu.ft./lb. = 12.5 pounds per minute
Displacement compressor does not change, but it pumps less pounds per unit of time. Therefore, its capacity falls. This is because its volumetric flow rate remained unchanged, while its mass flow decreased as a result of changes in system conditions. If everything else in the system remains unchanged, it will produce approximately 12% power reduction.
If the load up again, suction pressure in the evaporator will grow, causing increased mass flow rate. It allows to increase the productivity of the compressor and system, which is now, of course, "Riding with the load."..
|
Refrigeration 
