Compressor capacity and performance. Compressor capacity control
And with adjustable output Cooling unit this division in the compressor may be monitored to reduce or increase the refrigerant mass flow rate. The concept of compressor modulation of the flow that allows to increase productivity in two ways. First, when using the high-performance compressors and power decrease to prevent an increase in the mass flow rate refrigerant at high ambient temperatures, the COP in higher external conditions can be greatly increased. Reliability is increased by reducing the load on the compressor. The second performance improvement by changing the design of the system strategy. Ordinary heat pumps designed for the cooling load, so comfortable air conditioning is obtained. With capacity control compressor of the heat pump may be calculated on the large heat capacity, thereby having a lower balance point and the elimination of some Autonomous heating.
Then, through performance management, which is part of the concept, the capacity of the installation process of cooling can be controlled to ensure the proper management of comfort.
One method of capacity control system is often used today hot gas bypass. Hotgas-around, where the disposal of gas from the compressor vented back to the suction side of the compressor, is easy transition on most systems, but have catastrophic consequences in terms of energy saving, as the capacity is reduced without reducing the compressor, and is probably best avoided. Other possible potential control methods fall into three main categories:
1. Cruise control. Cruise control can be performed on a continuous or step by step. Continuously variable speed control is one of the most effective performance management techniques, and it offers good controls about 50% of the nominal speed of normal compressors. More than 50% speed reduction is unacceptable due to the lubrication of compressors. Continuously variable speed control also costly process, although not necessarily overly expensive, you can replace some of the usual starting the engine controls and, therefore, reduce the cost increment. Stepwise control of speed, as this is achieved, for example, using multipoled motors and switches the number of active poles, is another alternative. It may be possible to achieve a satisfactory performance improvements using a finite number stepped changes to vary productivity of the compressor. Step control is less expensive than continuously variable speed control, but also limited to 50% of the nominal speed of rotation of the compressor due to lubrication requirements. Also, a step change in load compressor can put a high load on the components of the compressor.
2. Clearance volume control. This requires significant additional amount of clearance to achieve the amount of flow reduction is desirable. For example, to reduce the mass flow rate of 50%, the volume must be equal to about half of the working volume, adding significantly the volume of compressor. Moreover, the large amount of residual masses causes unacceptably high discharge temperature with large amounts of flow reduction. For this reason, the clearance volume control is much less attractive than some other types of control.
3. Control valve. Suction valve discharge, compressor capacity control method is often used in large air conditioning and refrigeration systems, to reduce the cooling capacity, if the load decreases, you can achieve some energy savings, but has several disadvantages. The unloading, the intake valve one or more cylinders is open, so that the gas will be pumped into and back out of the cylinder through the valve without compression. Significant losses may occur due to the re-regulation through the suction valve. In addition, stepped cylinder unloading causes uneven stress on the crankshaft, and provides insufficient, if not totally unacceptable, control in small compressors. The method, however, is relatively inexpensive. Two new methods compressor flow regulation through the control valve the end suction valve closing and the early suction valve closing. In the late intake valve closing again takes on the control of losses by injection of the gas into the suction valve for part of the stroke. At the end of the valve closing, however, gives a more eligible, more smooth control than all of the exhaust valve. Currently, however, the method is limited to 50% reduction in power and large compressors low speed. At the beginning of the suction valve closing excludes losses as a result of gas throttling back of suction valves. Instead, foot valve, or additional valve slightly upstream of the intake valve is closed early on the intake stroke, limiting the amount of gas taken. The gas inside the cylinder extends, and then gas, resulting in significantly lower losses. Stepless capacity control in a broad range possible with the early closure of the valve approach. At the beginning of suctionvalve closing approach requires the greatest development potential management methods discussed above, but it also promising is one of the most efficient and cost-effective approaches.
Control of performance under a variety of loads, to ensure maximum efficiency
There are several ways to meet varying loads, each with a different degree of efficiency, as described below.
Building 1. One large compressor. It can't meet with variable load, and the results of its loss of productivity and decrease of performance when at part load.
Case 2. One large compressor with built-in management capacity. This is a good way to meet with variable load until the load remains above 50%.
Case 3. Three small compressors (two identical power and capacity management). This allows for quite close to the corresponding demand.
Building 4. Three small compressors of different capacity. This is a good way to meet with variable load. The purpose of mix and match different load sequence with control.
Building 5. Three compressors with the parallel control. This is often, but not always recommended due to the nonlinear input power supply with the power from turning down. For example. 180% power (i.e. 3) at 60%), it requires -240% of electricity due to inefficiency, which bring additional input about 60%.
In the Case of 6. Three compressor (two and one-off)- compressor is used at 100%, and another is to trim the exact demand (for example, 80% in the previous case), giving 180% capacity with 188% energy (22% savings more than in the previous case).
In choosing one of the above cases, the two main criteria of demand for electricity and budget. Please note that the load profile must be available to choose the best compressor option. Various options must be mapped into the most common operating conditions, and the entire load range. Efficiency of different variants, varies greatly and there is no hard and fast rule select the best solution. Switch compressor off to reduce the capacity of the system is the most efficient way of meeting a reduced load. Efficiency of the compressor for built-in control performance is always lower than when it is operating at full load. Efficiency of different methods of power control varies. In General. any method that recirculates the compressed gas into the compressor suction very poor. When considering compressors control, we must compare the parameters accurately. Compressors often oversized for the application because many safety factors used in calculating the load. This should be kept to a minimum, as too many compressors often work with less power factor.
Regardless of the setting selected in accordance load, control compressors important. Management strategy should be aimed at:
Select the most efficient mix of compressors to meet the load,
To avoid working on the built-in performance monitoring, when possible, and
Avoid low suction pressure when possible.
Selection of compressors in various sizes and developing effective strategies to control a loop them exactly match the most common loads often the most effective option...