TodayVapor-Compression RefrigerationPsychrometrics

TomorrowTeam Project

Wed – Test 3 (Entropy and Refrigeration)

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

WQ

COP

inQ

RefrigerationApplying Conservation of Energy…

12

41

21

32

14

0)(

0)(

0)(

hhhh

COP

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

Refrigeration

Cond

Comp

Qout

Win

Fermenting Room

Lagering Cellar Cooler

Hop Storage Cooler

Flash Tank

EvaporatorSecondary Refrigerant

Storage Tank

Wort Cooler

Fermenting Vessels

Green Beer Chiller

Beer Chiller

Pasteurizer

Yeast Tanks

Air Conditioning

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 − h4

h2 − h1

“Calculation of Performance using Mollier”

• Find the COP of a refrigeration plant when the evaporation temperature is -15C and the condenser temperature is 30C.– hsatvap @ -15C = 1426 kJ/kg– hsatliq @ 30C = 323.1 kJ/kg– hafter comp = 1662.4 kJ/kg– hbefore comp = 1426 kJ/kg

Refrigeration ExampleAn ideal vapor-compression refrigeration cycle

using ammonia operates between the pressures of 14 and 2 bar. The system cools a secondary refrigerant at a rate of 25 kW.

(a) Determine the mass flow rate of refrigerant.

(b) Determine COP of the system.(c) Determine the power consumed by the

compressor, in kW

Typical Manufacturers Performance Curves

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.

For a 10 ton capacity refrigeration system, the pressure of the refrigerant in the evaporator is 210 kPa, whereas in the condenser it is 750 kPa, If ammonia (R-717) is used under saturated conditions, calculate the theoretical power required to operate the compressor.

For a 10 ton capacity refrigeration system, the pressure of the refrigerant in the evaporator is 210 kPa, whereas in the condenser it is 750 kPa, If ammonia (R-717) is used under saturated conditions, calculate the theoretical power required to operate the compressor.