5-Co2 Systems

Question 1

What er the typical CO2 characteristics?

Answer 1

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

Question 2

What are the advantages of transcritical Co2 system

Answer 2

Provides high temperature lift, lower temperature difference in gliding condensing temperature means less energy loss

Question 3

What is the pressure range typically for LP side of general Co2 system?

Answer 3

26-41 bar

Question 4

What is the pressure range typically for HP side of general Co2 system?

Answer 4

61 bar

Question 5

What is the pressure range typically for transcritical side of general Co2 system?

Answer 5

81 bar

Question 6

Why is the pressure ratio usually lower for R744 circuits?

Answer 6

The general pressure of the circuit is high. A certain pressure lift will then result in a lower pressure ratio for R744 circuit

Question 7

What is the effect of combined low swept volume and low pressure ratio for R744?

Answer 7

This results in a higher efficiency for the compressor.

Question 8

Why does does R744 have a low temperature drop per pressure loss?

Answer 8

It is due to R744’s steep saturation curve

Question 9

What is the temperature and pressure for the triple point of R744?

Answer 9

T=-56°C and p=5,18 bar

Question 10

What is the temperature and pressure for the critical point of R744?

Answer 10

T=31,1°C and p=73,8 bar

Question 11

What are the main difference between R744 and ammonia LT applications?

Answer 11

Co2 system has: Smaller diameter pipes and larger flow velocities in HX and plate freezing temperature can be below -50°c

Question 12

What thermodynamic properties makes liquid R744 a good WF?

Answer 12

Good thermal conductivity, low viscocity and high specific heat capacity

Question 13

What problems follow R744’s high volume expansion when heated?

Answer 13

Containers must have a low liquid fill that allows for the expansion, risk of producing dry ice when safety valves open.

Question 14

What important effect does the pressure lift to HP side transcritical R744 system have?

Answer 14

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.

Question 15

What effect does the temperature of the evaporator have on the COP of R744 system?

Answer 15

An decrease of evaporator temperature will increase COP of R744 regardless of the high side pressure.

Question 16

What does a cold finger unit?

Answer 16

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.

Question 17

What is the difference between an indirect system and cascade system?

Answer 17

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.

Question 18

When do we use low-pressure receiver in R744 circuit?

Answer 18

When we want to have a low liquid level on low pressure side

Question 19

When do we use intermediate pressure collector in R744 circuit?

Answer 19

When we want an adjustable liquid level on medium pressure side

Question 20

When do we use low-pressure receiver and intermediate pressure collector in R744 circuit?

Answer 20

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.

Question 21

When do we use intercooling in R744 circuit?

Answer 21

When we have a large pressure lift for compressors. Intercooling between two compressor stages will increase compressor efficiency.

Question 22

When do we use parallell compression in R744 circuit?

Answer 22

When we want to be able to regulate the compression load

Question 23

What are the advantages of intercooling?

Answer 23

Reduced total compressor work and increased equipment lifespan

Question 23

Explain what an injector does

Answer 23

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.

Question 24

What are three different heat sinks that transcritical R744 system can be used for?

Answer 24

Heating water, heating rooms and melting ice

Question 25

Why has transcritical R744 a much higher COP than conv. HP?

Answer 25

Lower avg. refrigerant temp, lower pressure ratio, gliding temperature and high heat transfer due to properties of R744

Question 26

What is the ideal situation for achieving large energy savings with a trans critical R744 system?

Answer 26

Largest energy demand is hot water, medium temperature can be used for space heating and use lowest possible temp for gas cooler

Question 27

Mention some general advantages with R744 system besides operational performance+

Answer 27

1 Co2 equivalent, natural gas, non-flammable and non-toxic