Lecture 10 - Refrigeration Cycles 2 Flashcards
Give COPHP in terms of COPR
COPHP = COPR + 1
What is the desired output of a heat pump?
QH (heat out)
What can be used as heat source for a heat pump?
Technically anything - but the lower it is, the more difficult and expensive producing work becomes
What does a heat pump do?
Takes low grade heat, usually extracted from the ground at low temperature. Heat pump is used to raise it to a usable temperature
Give the factors that affect heat pump performance
Capacity and efficiency fall significantly at low source temp
Must be sized for the largest heat load required
Water vs air as heat source
Air systems:
-May require supplemental heating
-Can have frosting problems at high humidity - forms on evaporator coils, reducing heat transfer
Water (or soil)
-More constant temperatures
-If too much is extracted ground can freeze - only remove heat that can be replaced
Give the advantages of a heat pump as heating system
- Same components a refrigerator
- Design a reversible system to both heat and cool
- Free air conditioning
How does combined heating/cooling work?
Accomplished by reversing valve
Switches condenser
How does combined heating/cooling work?
Accomplished by reversing valve
Switches condenser and evaporator around
More practical for high cooling low heating loads
How do heat pumps compare with resistance heaters and gas boilers?
1/3 of the energy, C02 of resistance heater
3/4 of the c02 of gas boiler
Why dont we use heat pumps for domestic heating?
- Large area required
- Lower efficiencies - when taking losses into account
- Fluctuations in demand - not stable
If only electricity is available is a heat pump a good option
yes
Can we use a refrigerator as AC?
No - dumping heat into same place trying to take heat from - one heat source
Refrigeration requirement of heat pump
reach temp lower than required cooling temp
temp higher than the temp where you want to dump the heat
Can water be used as a working fluid?
Yes, but it would be very inefficient
Give options for working fluid of heat pump
Ammonia - limited to below 80C and very toxic
C02 non toxic natural substance low COP usually used in cascade
What working fluid does industry like and why?
Ammonia
Low cost and high COPs
However it is toxic
What was the issue with early refrigerants?
Give examples of early refrigerants
Highly toxic - sulphur dioxide, ethyl chloride and methyl chloride
Give the history of Freons
Frigidaire corp - requested GM to develop new refrigerant in 1928
Come up with CFCs - most suitable Freon
Wide Scale use in aerosols, foam insulation and as solvents
Ozone crisis caused new look
Give types of refrigerants (organic and inorganic)
Inorganic - ammonia, C02 sulphur dioxide
Organic - methylchloride, trichlorodifluoromethane, dichlorofluoromethane, fluoromethanes and others
For heat flow at reasonable rate, what should the difference in temperatures be between refrigerant and fluid?
5-10 degrees
Refrigerant critical temperature
greater than condenser temperature - approach carnot cycle and achieve high COP
Refrigerant Freezing temperature
Low - liquid only in evaporator - no freezing
Refrigerant saturation pressure
above atmospheric - avoid air leaks into system
Refrigerant Evaporation enthalpy
High - Reduces mass flow rate
Refrigerant Specific Volume
Low - Reduces compressor work and system size
Refrigerant Stability
Good - Both pure substances and mixtures
Refrigerant Thermal Conductivity
High - Good heat transfer rates
Refrigerant Solubility
Low - avoid water contamination
avoid oil contamination
Refrigerant Toxicity/irritancy
Low - avoid poisoning and convenient handling
Refrigerant Non Flammable
Safety in charging, handling and safety if leaks
Refrigerant Detectability
Good - for tracing leaks
Refrigerant Ozone Depletion
None
Refrigerant Cost
Low
Why was freon so good?
Met all the requirements of a refrigerant apart from ozone layer
Replaced CFCs with HFCs
What is the gas refrigeration systems?
Reverse brayton cycle
What is the gas refrigeration cycle used for?
Aircraft cooling
What is an absorption refrigeration cycle?
A cycle that uses solar energy converted as a compressor
