Four factors influencing the pressure coefficient
In the suction process of air compressor, because the spring force of air valve should be overcome when the suction valve is opened, and when the gas flows through the air valve, there is a certain flow resistance due to the small channel section and high flow speed, so the pressure of the gas in the cylinder in the suction process is always lower than the pressure in the suction pipe.
The pressure coefficient represents the influence degree of the intake resistance loss on the compressor gas transmission.
The ratio of the remaining working volume V2 of the cylinder to the original suction volume V1 is called the pressure coefficient, except for the volume △ V2 lost due to the intake resistance in the actual working process.
The factors influencing the pressure coefficient are as follows:
1. Structure of air valve.
The smaller the channel section of the valve, the greater the resistance loss. When the mass of the valve plate is large and the spring force of the air valve is large, the resistance loss increases, so the λ p value decreases.
Attention should be paid to the design: these two influencing factors are mutually restricted, and good results can be achieved only by combining both aspects.
2. Speed of air flow through the valve.
The pressure loss is proportional to the square of the air velocity.
The degree of Freon steam is much larger than that of ammonia steam, so when designing Freon compressor, it is necessary to consider that the channel section of air valve is larger, so that the speed of air flow passing through the valve is reduced, so as to reduce the pressure loss.
3. Effect of vapor pressure P1.
When P1 decreases, λ p value also decreases. Therefore, for compressors working at low temperature, the spring force of the air valve should be appropriately reduced to reduce △ P1.
For example, the low-pressure stage of a single and two-stage machine adopts a soft air valve spring.
4. The influence of relative clearance volume C.
It has been analyzed in the previous volume coefficient and will not be described here.
Influence of heat exchange and temperature coefficient
In the process of suction, the suction gas is continuously heated by the various wall surfaces contacted, which makes the temperature of the suction gas increase and the specific volume increase, thus reducing the suction gas volume. Assuming that the lost suction volume is converted to the volume of △ V3 in the suction state, the actual suction volume decreases from V2 to v3.
The heat exchange between suction gas and wall is a complex process, which is related to the type of refrigerant, compression ratio, cylinder size, compressor speed, cylinder cooling and other factors. The value of λ 1 is usually calculated by empirical formula.
The temperature coefficient reflects the effect of gas temperature rise on the suction capacity. The closer the value is to 1, the smaller the loss is.
It is the loss caused by the reduction of the amount of inhaled gas, so it is called invisible loss report.
The main way of compressor leakage is that there is no tightness between the piston ring and the cylinder wall, the sealing surface of suction and exhaust valves is not tight, or the suction and exhaust valves are not closed in time, so that the refrigerant gas leaks from the high pressure side to the low pressure side, resulting in the decrease of gas transmission.
The leakage is related to the manufacturing quality, wear degree, valve design, pressure difference and other factors of the compressor.
Due to the improvement of modern processing technology and product quality, the leakage of compressor is very small, so λ 1 value is generally very high, and λ 1 = 0.97 ~ 0.99 is recommended.