Lecture 9 - Refrigeration Cycles 1

Question 1

What is the difference between refrigerator and heat pump?

Answer 1

Refrigerator - want cold refrigerated space, heat to warm environment
Heat pump - want warm house, heat from cold environment

Question 2

What is refrigeration?

Answer 2

process of making heat flow from cold to hot

which requires external work

Question 3

What is the performance of a refrigerator and heat pump measured in?

Answer 3

Coefficient of Performance (COP) - can be greater than one

Question 4

COP for refrigerator

Answer 4

Cooling Effect/Work input = QL / Wnet

Question 5

COP for heat pump

Answer 5

Heating Effect/Work Input = QH / Wnet

Question 6

What is cooling capacity?

Answer 6

tons of refrigeration/cooling

capacity of refrigeration that will freeze 1 ton of liquid water at 0C into ice at 0C in 24 hours

Question 7

Calculate Wnet in for 1 ton of cooling capacity of water, COP of 3

Answer 7

Heat capacity of water at 1 atm 0C = 333kJ/kg
Q = 333x10^3 x 1000kg / (24hours x 3600 sec) = 3.85kW
Wnet = Q/COP = 3.85/3 = 1.28 kW

Question 8

What is the ideal refrigeration cycle TS diagram?

Answer 8

Reversed Carnot

Question 9

Which components are in the ideal refrigeration cycle block diagram?

Answer 9

Evaporator
Compressor
Condenser
Turbine

Question 10

Give the 4 stages of reversed carnot cycle and two key aspects of the cycle

Answer 10

Max possible efficiency, reversible cycle
1-2 Constant temp and pressure heat addition
2-3 Isentropic Compression
3-4 Constant temp and pressure heat rejection
4-1 Isentropic Expansion

Question 11

How is the COPr for Carnot cycles calculated?

Answer 11

TL/TH-TL

Question 12

Give the problems with reversed carnot cycle stages

Answer 12

1-2 3-4 are ok
2-3 requires specical hardware to compress two phase fluid
Expansion of high liquid content fluid is difficult

Question 13

What replaces the turbine in the refrigeration cycle, why, and what effect does this have?

Answer 13

Throttle valves - therefore cycle is no longer reversible

Don’t use turbines as mixture would be difficult to do and would make very loud

Question 14

Draw a schematic of a household refrigerator

Answer 14

Evaporator coils
Capillary tube
Kitchen air
Condenser coils
Compressor
QH and QL

Question 15

What are the resulting first law equations for an ideal vapour refrigeration cycle?

Answer 15

Compressor s = const Win = m(h2 - h1)
Condenser P = const QH = m(h2 - h3)
Throttle Valve Wnet = 0 Qnet = 0 h4 = h3
Evaporator P = const  QL = m(h1-h4)

Question 16

Give the key point for when working with charts and tables

Answer 16

Different tables have different datums - this is okay as working in done in differences

Question 17

Can a binary cycle be used in refrigeration? What are the key points of this?

Answer 17

Yes, called a cascade system - use different refrigerants to best utilise their temperature range

Question 18

What is the benefit of a cascade system?

Answer 18

Decrease compressor work

Increase refrigeration capacity

Question 19

Give the mass balance of heat exchanger in cascade system

Answer 19

ma(h5-h8) = mb(h2-h3)

Question 20

Where are multipurpose systems used?

Answer 20

Where refrigeration is needed at more than one temperature

Question 21

Give ways to improve performance of vapour compression refrigeration system

Answer 21

Cascading - allows very low temperatures and increased COP

Multistage compression with regenerative cooling