Refrigeration efficiency of expander

Nitrogen Compressor Exporter Explains Expander Refrigeration Efficiency:

Refrigeration is an extremely important component of cryogenic air separation
The precondition of open heat pump distillation is also the necessary condition for realizing stable operation of cryogenic air separation distillation (i.e. cooling capacity balance). Before the expansion refrigeration process plan, the cooling capacity of the cryogenic air separation comes from the so-called isothermal enthalpy difference, so the refrigeration efficiency of the cryogenic air separation under the process plan is very low! Now the expansion refrigeration process plan is already the refrigeration process plan of the cryogenic air separation standard. Next, we will discuss the refrigeration efficiency of the expander, which is the most basic problem in the energy consumption accounting of the cryogenic air separation.
The gas is compressed to a certain pressure at room temperature (it is segmented adiabatic compression and then cooled to room temperature), and then enters the expander for expansion refrigeration after heat exchange between the heat exchanger and the working medium reflux gas of the expansion refrigeration cycle! After generating enthalpy drop and cold energy, the cold capacity and cold energy are supplied to the cold capacity and cold energy receptor, and then enter the next cycle after the heat exchanger exchanges heat with the working medium of the positive flow expansion refrigeration cycle. In order to simplify the calculation process, the gas is assumed to be an ideal gas (that is, there is no so-called isothermal enthalpy difference). At the same time, it is assumed that the positive reflux resistance, cooling dissipation loss and heat exchange temperature difference are all zero, that is, under the limit engineering conditions. Then, the influence of engineering conditions, resistance loss of positive return flow, temperature difference of heat exchange and cooling loss on expansion refrigeration efficiency is discussed.
Under such assumption, it is clear that the inlet temperature of the expander is the boiling point (dew point) temperature of the working medium in the positive flow cycle! If the thermal insulation efficiency of the expander is 100% (i.e. reverse expansion process), then the pressure of the positive flow circulating working medium at the inlet of the expander can all be converted into the enthalpy drop, cold energy and expansion output work of the circulating working medium! The ratio of the output work and the pressure energy of the circulating working medium is called the maximum work coefficient, or the maximum refrigeration coefficient. There is no doubt that the higher the inlet temperature of the expander (the higher the pressure of the working medium corresponding to the positive flow cycle), the greater the maximum work coefficient and the maximum refrigeration coefficient. In the expansion process of cryogenic air separation, the inlet temperature of expander is 100-150k, and the maximum work coefficient or refrigeration coefficient is 0.3-0.5.
The effective energy analysis and effective energy efficiency calculation of the energy system were very confused! Generally, the ratio of the sum of the effective energy of the import and export of the system is taken as the effective energy efficiency of the system. In essence, this is the effective energy input-output rate, especially when calculating the effective energy efficiency of the subsystem, which will lead to incomprehensible results. Now, the country has formulated technical guidelines for exergy analysis of energy systems. The most important breakthrough is the definition and calculation formula of system target exergy efficiency. The following calculation of effective energy efficiency is based on this.
Turbocharging process is generally adopted for expansion refrigeration. Generally speaking, the isothermal efficiency of turbocharger is lower than that of air compressor. For the convenience of calculation, it is assumed that the isothermal efficiency of both is 70%! " K5 M4 Z! x9 s/ A4 B6 N2 c
Calculate the effective energy efficiency of the system. The system can be divided into three parts from large to small. One is the whole system, the other is the medium system excluding turbocharger, and the third is the small system excluding air compressor and turbocharger. In the calculation, the compression power consumption of the compressor is assumed to be 1kWh, and the isothermal efficiency of the air compressor and the turbocharger is 70%. If the thermal insulation efficiency of the expander is 100%, that is, fully reversible thermal insulation expansion, then under extreme engineering conditions, when the thermal insulation efficiency of the expander is 100%, the expansion cooling efficiency of the small system is 100%! The expansion refrigeration efficiency of the middle system and the whole system is 70%, that is, the isothermal efficiency of the air compressor and turbocharger. It is independent of the expander inlet temperature, maximum work coefficient and refrigeration coefficient.
Now discuss the influence of the adiabatic efficiency of the expander on the expansion refrigeration efficiency. Assume that the maximum work coefficient (i.e. the maximum refrigeration coefficient) is b, and the value of b is 0.3! When the thermal insulation efficiency of the expander is 85%, the expansion refrigeration efficiency of the large system is 70% X (1-0.3) X85%/1-0.3X0.7X85%=50.5%. If you consider that when the thermal insulation efficiency of the expander is reduced from 100% to 85%, the b value will become larger, and the expansion refrigeration efficiency has been determined to be within 50%! The effective energy efficiency of the system in is 70% X (1-0.3) X85%/1-0.7X0.3X85%=0.4165/0./0.8215=50.5%! As the actual cold energy value is slightly less than the above molecular calculation value, the expansion refrigeration efficiency is determined to be within 50%. Whether the turbocharger is included does not affect the expansion refrigeration efficiency. This is because the isothermal efficiency of the air compressor and the turbocharger is identical. If the isothermal efficiency of the turbocharger is lower than that of the air compressor, the expansion refrigeration efficiency of the large system will be lower than that of the medium system. Of course, this does not mean that the adoption of the turbocharging process scheme is unfavorable, which is another problem. Effective energy efficiency of small system=70% X (1-0.3) 85%/70% - 70% X0.3X85%=0.4165/0.5215=80%! " _$ j+ `7 P% {* L' f
If other parameters remain unchanged and the adiabatic efficiency of the expander drops to 80%, the expansion refrigeration efficiency of the large system will be 47% and that of the small system will be 74%!
Now let's discuss the influence of the maximum work coefficient or the maximum refrigeration coefficient on the expansion refrigeration efficiency. When the b value is 0.9 (that is, the inlet temperature of the expander is close to normal temperature), the isothermal efficiency of the air compressor turbocharger is 70%, and the thermal insulation efficiency of the expander is 85%. The expansion refrigeration efficiency of the large system is 11%! The expansion refrigeration efficiency of small system is only 32%! It is much lower than the cooling efficiency of heat pump by 60%! When the b value is 0.5 (that is, the inlet temperature of the expander is about 150K) and other parameters remain unchanged, the effective energy efficiency of the expansion refrigeration of the large system is 42.5%!
Now, we will discuss the influence of positive reflux resistance on expansion refrigeration efficiency under engineering conditions. Due to the existence of positive backflow resistance, first, the inlet pressure of the expander is lower than the outlet pressure of the air compressor, and second, the expansion terminal pressure of the expander cannot reach the normal pressure. We estimate the impact on the expansion refrigeration efficiency based on the pressure loss of 10% and 20% (under the actual engineering conditions of the low-temperature expander, the pressure energy loss is generally within this range). When the pressure energy is lost by 20% due to the positive reflux resistance, the effective energy efficiency of the large expansion refrigeration system is 39% (50% without the positive reflux resistance loss). When the pressure energy is lost by 10%, the effective energy efficiency of the large expansion refrigeration system is about 45%.
We discussed the expansion refrigeration efficiency above, but the cryogenic gas (liquefaction) process is a combined process scheme of heat pump expansion refrigeration, and the actual liquefaction efficiency of cryogenic gas will be discussed later.