5-Co2 Systems Flashcards
(28 cards)
What er the typical CO2 characteristics?
Low temperature and pressure at triple point, generally high pressure in systems, high thermal conductivity, high mass heat capacity and high thermal expanison coefficient in liquid phase
What are the advantages of transcritical Co2 system
Provides high temperature lift, lower temperature difference in gliding condensing temperature means less energy loss
What is the pressure range typically for LP side of general Co2 system?
26-41 bar
What is the pressure range typically for HP side of general Co2 system?
61 bar
What is the pressure range typically for transcritical side of general Co2 system?
81 bar
Why is the pressure ratio usually lower for R744 circuits?
The general pressure of the circuit is high. A certain pressure lift will then result in a lower pressure ratio for R744 circuit
What is the effect of combined low swept volume and low pressure ratio for R744?
This results in a higher efficiency for the compressor.
Why does does R744 have a low temperature drop per pressure loss?
It is due to R744’s steep saturation curve
What is the temperature and pressure for the triple point of R744?
T=-56°C and p=5,18 bar
What is the temperature and pressure for the critical point of R744?
T=31,1°C and p=73,8 bar
What are the main difference between R744 and ammonia LT applications?
Co2 system has: Smaller diameter pipes and larger flow velocities in HX and plate freezing temperature can be below -50°c
What thermodynamic properties makes liquid R744 a good WF?
Good thermal conductivity, low viscocity and high specific heat capacity
What problems follow R744’s high volume expansion when heated?
Containers must have a low liquid fill that allows for the expansion, risk of producing dry ice when safety valves open.
What important effect does the pressure lift to HP side transcritical R744 system have?
Because we have a gliding condensation temperature we get a pinch point problem. Higher pressure lift gives drastically larger temperature lift of the cold medium.
What effect does the temperature of the evaporator have on the COP of R744 system?
An decrease of evaporator temperature will increase COP of R744 regardless of the high side pressure.
What does a cold finger unit?
When there is a pressure build up the cold finger relieves this pressure by extracting some gas WF. The cold finger liquifies it and recycles the WF back to the cycle.
What is the difference between an indirect system and cascade system?
In principle on could have used the WF of the secondary system in the indirect system all the way but for some reason (f.eks safety) we want to avoid that. For the cascade system the secondary system is used as an energy source for the primary system. This is necessary if the primary system needs an evaporator temperature higher than ambient temperature.
When do we use low-pressure receiver in R744 circuit?
When we want to have a low liquid level on low pressure side
When do we use intermediate pressure collector in R744 circuit?
When we want an adjustable liquid level on medium pressure side
When do we use low-pressure receiver and intermediate pressure collector in R744 circuit?
Low pressure receiver is used when we focus on the efficiency of the evaporator. Intermediate pressure receiver is used for increasing the efficiency of the two staged compressor, and they can be combined.
When do we use intercooling in R744 circuit?
When we have a large pressure lift for compressors. Intercooling between two compressor stages will increase compressor efficiency.
When do we use parallell compression in R744 circuit?
When we want to be able to regulate the compression load
What are the advantages of intercooling?
Reduced total compressor work and increased equipment lifespan
Explain what an injector does
An injector uses the kinetic energy from the gas to increase the temperature of the liquid. The liquid is diffused into a low pressure chamber and then the gas is injected. The gas will then drive a fairly large proportion of the liquid to gas state.
