Compressor Part Load Operation

Refrigeration systems must be sized for peak design conditions, even if these conditions occur only several hours in a year. Most of the time, conditions are in the middle regions of the cooling load curve. There are several factors that affect performance under varying loads, including driver efficiency, compressor volumetric efficiency, ambient temperature, the relationship between heat exchange surface area and load, and the characteristics of the refrigerant.

To a certain extent, compressors automatically reduce capacity as load decreases. This is commonly referred to as "riding with the load." While a compressor operating at constant speed without constricting refrigeration flow maintains constant volume flow, mass flow rate varies with operating conditions. It is the mass flow rate that determines the capacity to transfer heat. Mass flow, which is a function of volumetric flow and specific heat, can be expressed as:

„ Volumetric flow rate

Specific volume

As loads decrease, less heat is absorbed into the evaporator, so less refrigeration is vaporized. The suction pressure drops, the saturation temperature of the refrigeration is reduced, and the specific volume increases. Since the volumetric flow is constant, the mass flow decreases, meaning less refrigeration mass is being pumped per unit of time. As a result, capacity decreases. As load is increased, the process is reversed and mass flow and, therefore, capacity, increases.

As suction pressure and temperature decrease, partial freezing of the moisture that collects on a refrigerant-to-air evaporator coil may result. The frost decreases the amount of air that passes through the coil, which lowers the suction pressure and temperature even further. This blocks airflow and can eventually damage the compressor.

In order to control operation at lower loads, compressors are designed to "unload" under partial-load conditions. Each compressor design has a different technique for unloading or reducing power draw as loads decrease. Being positive displacement machines, as cooling load requirements decrease, reciprocating and screw compressors reduce displacement. Centrifugal compressors, which are dynamic machines, reduce output by reducing the effect of centrifugal force.

As load decreases, heat dissipation requirements fall and suction pressure and temperature are usually lower than they are under fully loaded conditions. However, because the heat exchange surface area is constant, the heat exchanger is effectively oversized and, therefore, more efficient under partial load. If load falls as a result of decreased outside air temperature, condenser water temperature also falls. Hence, refrigeration cycle efficiency is improved. Counteracting these improvements is usually a reduction in compressor isentropic efficiency.

Typically, in the upper third of the load curve at constant speed, power requirement decreases and cycle efficiency increases as loads decrease. In this load range, the negative impact of compressor volumetric efficiency loss is usually more than compensated for by other efficiency gains. As loads fall further, system performance begins to decrease as the effect of degrading compressor volumetric efficiency begins to outweigh other system efficiency enhancement factors. In the bottom third of the load curve, system performance begins to fall off sharply. While decreasing ambient temperatures (assuming a relationship between load and ambient temperature exists in the particular application) continue to partially off-set compressor volumetric efficiency degradation under low loads, most systems have limitations as to the minimum acceptable condenser temperature.

Typically, as loads fall below 15% of full load, the power requirement rate may be as much as double the full-load design rating, even at reduced ambient temperature conditions. In this operating range, driver thermal efficiency degradation also becomes a contributing factor. For this reason, systems designed for frequent operation in lower load ranges may be designed with multiple compressors to reduce system performance degradation under low-load operation.

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