Upload
rafe-gaines
View
216
Download
0
Tags:
Embed Size (px)
Citation preview
Objectives
• Learn about refrigerants, compressors, and expansion valves (Ch. 4)
• Introduce heat exchangers (ch.11)
Reciprocating Compressor
Reciprocating
• Piston compressing volume• PVn = constant = C
• For all stages, if we assume no heat transfer
• Can measure n, but dependent on many factors• Often use isentropic n in absence of better
values• R-12 n =1.07• R-22 n = 1.12• R-717 n = 1.29
Summary
• Many compressors available• ASHRAE Handbook is good source of more
detailed information• Very large industry
Expansion Valves
• Throttles the refrigerant from condenser temperature to evaporator temperature
• Connected to evaporator superheat• Increased compressor power consumption• Decreased pumping capacity• Increased discharge temperature
• Can do it with a fixed orifice (pressure reducing device), but does not guarantee evaporator pressure
Thermostatic Expansion Valve (TXV)
• Variable refrigerant flow to maintain desired superheat
AEV
• Maintains constant evaporator pressure by increasing flow as load decreases
Summary
• Expansion valves make a big difference in refrigeration system performance
• Trade-offs• Cost, refrigerant amount• Complexity/moving parts
In Addition….
• Toxicity• Flammability• Ozone-depletion• Greenhouse potential• Cost• Leak detection• Oil solubility• Water solubility
Refrigerants
• What does R-12 mean?• ASHRAE classifications• From right to left ←
• # fluorine atoms
• # hydrogen atoms +1
• # C atoms – 1 (omit if zero)
• # C=C double bonds (omit if zero)
• B at end means bromine instead of chlorine• a or b at end means different isomer
Refrigerant Conventions
• Mixtures show mass fractions
• Zeotropic mixtures• Change composition/saturation temperature as
they change phase at a constant pressure
• Azeotropic mixtures• Behaves as a monolithic substance• Composition stays same as phase changes
Inorganic Refrigerants
• Ammonia (R717)• Boiling point• Critical temp = 271 °F• Freezing temp = -108 °F• Latent heat of vaporization
• Small compressors
• Excellent heat transfer capabilities• Not particularly flammable
• But…
Carbon Dioxide (R744)
• Cheap, non-toxic, non-flammable
• Critical temp?
• Huge operating pressures
Water (R718)
• Two main disadvantages?
• ASHRAE Handbook of Fundamentals Ch. 20
Water in refrigerant
• Water + Halocarbon Refrigerant = (strong) acids or bases• Corrosion
• Solubility• Free water freezes on expansion valves
• Use a dryer (desiccant)
• Keep the system dry during installation/maintenance
Oil
• Miscible refrigerants
• High enough velocity to limit deposition• Especially in evaporator
• Immiscible refrigerants • Use a separator to keep oil contained in
compressor
• Intermediate
The Moral of the Story
• No ideal refrigerants
• Always compromising on one or more criteria
Air-liquid Tube heat exchanger
Plate heat exchanger
Heat exchangers
Air-air
Some HX (Heat Exchanger) truths
• All of the energy that leaves/enters the refrigerant enters/leaves the heat transfer medium
• If a HX surface is not below the dew point of the air, you will not get any dehumidification• Water takes time to drain off of the coil
• Heat exchanger effectivness varies greatly
Heat Exchanger Effectiveness (ε)
C=mcp
exchangeheatposible Maximum
exchangedHeat
Location B Location A
THout
TCin
TCout
THin
Mass flow rate Specific capacity of fluid
Example:What is the saving with the residential heat recovery system?
Furnace
72ºF
32ºF 72ºF
Outdoor Air
For ε=0.5 and if mass flow rate for outdoor and exhausted air are the same
50% of heating energy for ventilation is recovered!
For ε=1 → free ventilation! (or maybe not)
52ºF Exhaust
Gas
Combustion
products
Fresh Air
Air-Liquid Heat Exchangers
• Fins added to refrigerant tubes
• Important parameters for heat exchange?
Coil Extended Surfaces Compact Heat Exchangers
What about compact heat exchangers?
• Geometry is very complex
• Assume flat circular-plate fin
Overall Heat Transfer
Q = U0A0Δtm
Overall Heat
Transfer Coefficient
Mean temperature
difference
Heat Exchangers
• Parallel flow
• Counterflow
• Crossflow
Ref: Incropera & Dewitt (2002)
Heat Exchanger Analysis - Δtm
Heat Exchanger Analysis - Δtm
ocih
icoh
ocihicohm
tt
tt
ttttt
,,
,,
,,,,
ln
Counterflow
A
B
ABm
tt
ttt
ln
For parallel flow is the same
or
A
B
ABm
tt
ttt
ln