Post on 25-Dec-2015
Refrigeration Terms• Cooling Load, Cooling Capacity – Qin
• Compressor Load – Win
• Condenser Load – Qout
• Tons of Refrigeration – Rate of Heat Input• Refrigerant – The Fluid• Vapor-Compression Refrigeration• Heat Pump – Same Cycle, Use Qout
RefrigerationEfficiency = desired output / required inputDesired output = Heat removal from refrigerated
space (Qin)
Required input = Work input to compressorConservation of Energy: Qin + Win = Qout
COP can be > 1.0 = Cooling Capacity
in
in
W
QCOP
inQ
RefrigerationApplying Conservation of Energy…
12
41
21
32
14
0)(
0)(
0)(
hh
hhCOP
hhmW
hhmQ
hhmQ
in
out
in
Refrigeration
• Used when no other method of cooling is available
• Very expensive (40-60% of a brewery’s utility bill)
• Removal of heat from low T source to high T sink
Primary RefrigerantsAmmonia (R-717), R-12, R-134aSaturation temp < Desired application temp
2 to 8C Maturation tanks0 to 1C Beer Chillers-15 to -20C CO2 liquefaction
Typically confined to small region of brewery
Secondary RefrigerantsWater with alcohol or salt solutionsMethanol/glycol, potassium carbonate, NaClLower freezing temperature of waterLow-toxicity (heat exchange with product)Pumped long distances across brewery
Theory and the Cycle
Condenser
Evaporator
Compressor
Qout
Qin
Win
1
23
4
Refrigeration1-2: Constant entropy compression (s1 = s2)2-3: Constant pressure heat rejection (3 = sat liq.)3-4: Constant enthalpy throttling4-1: Constant pressure heat addition (1 = sat vap.)
Coefficient of Performance
• Describes how well a refrigeration plant is running
• Heat removed divided by energy input• COP increase with temperature difference
between source and sink
€
COP =QeWc
=h1 − h4h2 − h1
Refrigeration ExampleAn ideal vapor-compression refrigeration cycle
using ammonia operates between the pressures of 2 and 14 bar. The system cools a secondary refrigerant at a rate of 25 kW. Determine:
(a) The evaporator and condenser temperatures(b) The mass flow rate of refrigerant.(c) The COP of the system.(d) The power consumed by the compressor, in
kW
Typical Manufacturers Performance Curves
Chemical structure of refrigerants
Refrigerant R12, CF2Cl2
Demanded properties of refrigerants
• Today the preservation of the ozone layer is the first priority of refrigeration selection
How is the ozone depleted by CFC’s
The Nobel Prize in Chemistry 1995
The Royal Swedish Academy of Sciences has decided to award the 1995 Nobel Prize in Chemistry to Paul Crutzen, Mario Molina and F. Sherwood Rowland for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone.
Ozone Depletion Potential (ODP)
Compressor Types• Reciprocating – similar to piston pump• Good for full and part-load• Good speed control and smaller apps
• Screw – Single or Twin• Smooth operation, good for large apps• Good at full-load, poor at part-load
Dry Air Fin Condensers• Fluid in condenser does not contact cooling
fluid• High electricity costs for fans
Wet Evaporative Condensers• Fluid in condenser does not contact cooling
fluid• Water sprayed onto tubes to evaporate and
cool
Cooling Tower Condensers• A secondary fluid (water) sprayed• Air passes across water droplets, cools• Forced or induced draft, counter or cross• Cool water to heat exchange condenser
Condenser Selection Considerations• Ambient temperature (Air-fin?)• Ambient humidity (evaporation?)• Space, accessibility, maintenance• Electricity costs (air-fin)• Chemical costs (evaporative, tower)
Legionellosis or L. pneumophila• Major source cooling towers and evaporative
coolers• Name from 1976 meeting of American Legion
– killed 36 people• Kill by heating to 60oC or chlorine
Evaporators and Expansion Devices• Direct expansion with thermostat valve• Regulates flow of liquid being throttled into
evaporator• Diaphragm to balance pressure between liquid
in condenser and sum of evaporator and spring pressure
Evaporators and Expansion Devices• Flooded with level control• Level of liquid in reservoir (typically shell and
tube heat exchanger) controlled with variable throttle valve.