A capacity controlled refrigeration unit is a unit in which the compression ability of the compressor can be controlled to reduce or increase refrigerant mass flow rate. The concept of compressor flow modulation achieves improved performance in two ways. First, by using efficient compressor capacity reduction to prevent the increase in mass flow rate of refrigerant at high ambient temperatures, the COP at higher ambients can be significantly increased. Reliability will also be increased because of reduced load on the compressor. The second improvement in performance is realized by a change in system sizing strategy. Conventional heat pumps are sized for the cooling load so that comfortable air conditioning is obtained. With compressor capacity control the heat pump can be sized for a greater heating capacity, thereby having a lower balance point and eliminating some of the auxiliary heating. Then, via the capacity control which is inherent in the concept, the capacity of the unit during cooling can be controlled to achieve proper comfort control.
One method of system capacity control frequently in use today is hot-gas-bypass. Hotgas-bypass, where discharge gas from the compressor is vented back to the suction side of the compressor, is an easy retrofit to most systems, but is disastrous from an energy savings viewpoint because capacity is reduced without reducing compressor work, and is probably best avoided. Other possible capacity control methods fall into essentially three categories:
1. Speed control. Speed control can be done either continuously or stepwise. Continuously variable speed control is one of the most efficient methods of capacity control, and it offers good control down to about 50% of rated speed of normal compressors. More than 50% speed reduction is unacceptable because of lubrication requirements of the compressors. Continuously variable speed control is also an expensive process, though not necessarily prohibitively expensive, for it might be possible to replace some of the conventional starting controls with the motor controls and hence reduce the cost increment. Stepwise speed control, as achieved, for example, by using multipoled electric motors and switching the number of active poles, is another viable alternative. It might be possible to achieve satisfactory improvements in performance by using a finite number of stepped changes to vary compressor capacity. Step control is less costly than continuously variable speed control, but is also limited to 50% of rated compressor speed because of lubrication requirements. Also, step changes in load on the compressor could put high stresses on compressor components.
2. Clearance volume control. This requires substantial amounts of additional clearance volume to achieve the amount of flow reduction desirable. For example, to reduce the mass flow rate by 50%, the clearance volume must be equal to about half of the displacement volume, adding substantially to the bulk of the compressor. Moreover, the large amount of residual mass causes unacceptably high discharge temperatures with large amounts of flow reduction. For this reason, clearance volume control is considerably less attractive than some other types of control.
3. Valve control. Suction valve unloading, a compressor capacity control method often used in large air conditioning and refrigeration systems to reduce cooling capacity when load decreases, can achieve some energy savings but has a number of drawbacks. In unloading, the suction valve of one or more cylinders is held open so that gas is pumped into and back out of the cylinder through the valve without being compressed. Substantial losses can occur because of this repeated throttling through the suction valve. In addition, stepwise cylinder unloading causes uneven stresses on the crankshaft, and provides inadequate, if not totally unacceptable, control in smaller compressors. The method, is however, relatively inexpensive. Two newer methods of compressor flow regulation via valve control are late suction valve closing and early suction valve closing. Late suction valve closing again incurs the throttling loss by pumping gas back out of the suction valve for part of the stroke. Late valve closing, however, gives more acceptable, smoother control than complete valve unloading. At present, however, the method is limited to a maximum of 50% capacity reduction and to large low speed compressors. Early suction-valve closing eliminates losses due to throttling gas back out of the suction valves. Instead, the suction valve, or a secondary valve just upstream of the suction valve, is closed prematurely on the intake stroke, limiting the amount of gas taken in. The gas inside the cylinder is expanded and then recompressed, resulting in much lower losses. Continuously variable capacity control over a wide range is possible with the early valve closing approach. The early suctionvalve closing approach requires the most development of the capacity control methods discussed above, but it also holds promise for being one of the most efficient and inexpensive approaches.
Capacity control for varying loads to provide better efficiency
There are several ways to meet varying loads, each with different efficiency, as summarized below.
• Case 1. One single large compressor. This cannot meet variable load, and results in wasted capacity and lower efficiency when at part load.
• Case 2. One single large compressor with in-built capacity control. This is a good option to meet variable load as long as load stays above 50%.
• Case 3. Three small compressors (two same capacity and one with capacity control). This allows fairly close matching to demand.
• Case 4. Three small compressors with different capacities. This is a good option to meet variable load. The aim is to mix and match to varying load with sequence control.
• Case 5. Three compressors with parallel control. This is often used, but is not always recommended due to non-linear input power with capacity turn-down. For example. at 180% capacity (i.e. 3 at 60%), it requires -240% power due to inefficiencies, which bring an additional input of about 60%.
• Case 6. Three compressors (two are on and one is off)- In this case one compressor is used at 100%, and one is used to trim to exact demand (for example, 80% in the above case), giving 180% capacity with 188% power (22% saving over the above case).
In the selection of one of the above cases, two main criteria are power demand and budget. Note that the load profile must be available to select the best compressor option. Different options should be compared at the most common operating conditions as well as throughout the load range. The efficiency of the different options varies enormously and there is no hard and fast rule to selecting the best solution. Switching a compressor off to reduce the system capacity is the most efficient method of meeting a reduced load. The efficiency of a compressor operating on inbuilt capacity control is always lower than when it operates at full load. The efficiency of the different methods of capacity control varies. In general. any method which recirculates compressed gas back into the suction of a compressor is very poor. When considering compressors with capacity control, we should compare the options accurately. Compressors are often oversized for an application because so many safety factors are used when calculating the load. This should be minimized as oversized compressors often operate with a lower power factor.
Regardless of the configuration option selected to meet a load, the control of the compressors is important. The control strategy should be designed to:
• select the most efficient mix of compressors to meet the load,
• avoid operation on inbuilt capacity control when possible, and
• avoid operation at low suction pressures when possible.
Selecting compressors of different sizes and designing a good control strategy to cycle them to accurately match the most common loads is often the most efficient option.