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Outline
• Revision - the basics• Component operation• Factors effecting efficiency
– load reduction– operating conditions
• The low hanging fruit
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• Common principles• How a system works• Importance of operating conditions:
– evaporating temperature– condensing temperature– superheat– subcooling
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COSP
• What do you expect the COSP to be?
Typically …• LT ~ 0.5 to 1.5• HT ~ 2 to 4• AC ~ 4 to 6
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Saturated mixturelow pressure
Superheated gaslow pressure
Superheated gashigh pressure
Saturated liquidhigh pressure
Saturated mixturehigh pressure
Subcooled liquidhigh pressure
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Compressor
Most efficient if:• compression ratio low• suction superheat low• cooling adequate
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Evaporator
Evaporator effectiveness depends on ...• evaporating temperature / pressure
– size / condition of evaporator– temperature of cooled space / fluid
• degree of useful superheat• degree of subcooling
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Condenser
Condenser effectiveness depends on ...• condensing temperature / pressure
– size / condition of condenser– cooling medium– temperature of cooling medium
• head pressure control
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Superheat
Useful superheat• in evaporator• aim for ~ 5KNon useful superheat• in suction line• minimiseRefrigerant arriving at compressor must be
superheated (but not too much!)
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Subcooling
• 10K subcooling ~ 7% capacity increase– natural cooling or by subcooler– insulate liquid line when using a subcooler
• No subcooling ~ ??% capacity reduction– condensing temperature too low– liquid line passes through hot area– loss of refrigerant
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Expansion Valve
• Subcooled liquid at entry• Useful superheat setting important• TEV, minimum 6 bar pressure drop• EEV, minimum 4 bar pressure drop
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Central Plant Systems
• Several smaller compressors for better capacity matching to load
• Can be unevenly sized• Controlled on suction pressure
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Central Plant Systems
30
40
50
60
70
80
90
100
33 66 100
% loading
pow
er
screw withslide valve
Single 6cylinder
3 compressorpack
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An Efficient System ...
…has minimum load…has minimum temperature lift…uses an efficient combination of
components…is well controlled
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Load - Defrost
• Does the defrost work?
• Is it controlled correctly
• Defrost on demand can improve efficiency
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Load - Sec. Ref. Pumps
• Contribute twice to energy use– direct electricity consumption– heat into secondary fluid
• Variable Speed Drive – will reduce pump power, e.g. by 25%– will reduce heat load
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Load - Insulation
Cold store / cabinet insulationSecondary fluid pipe work insulation• ensure in good condition• ensure sufficient thickness• ensure well joined / sealedSuction lineLiquid line
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Temperature Lift
• Evaporating temperature• Condensing temperature• Control
1°C = 2% to 4%
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Evaporating Temperature
• Cooled space / fluid temperature- as high as possible
e.g. increasing frozen food store temperature from -25°C to -20°C saves over 10% (and reduces breakdown)
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Evaporating Temperature
• Ensure evaporator is clean
• Ensure evaporator is clear of frost
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Evaporating Temperature
• Refrigerant charge is critical– leak test– repair– re-charge
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Reduce Leak Potential
• Brazed joints• Minimum charge• Minimum pressures • Charge monitoring• Effective leak testing
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Condensing Temperature• Recommend replacement
– take opportunity to fit larger condenser
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Condensing Temperature
Non condensable gases increase condensing temperature
• Purge air / nitrogen– use an automatic (refrigerated) air
purger– portable versions available
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Control
• Highest possible evaporating temperature– thermostat / controller (LP) setting
• Lowest possible condensing temperature– minimum head pressure control
• Appropriate useful superheat– TEV / EEV adjustment
• Optimum compressor strategy
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Low Hanging Fruit
• Cold store air change load reduction
• Auxiliary load reduction• Reducing refrigerant leaks• System control• Re commissioning• Maintenance
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Air Change Reduction
• Heat and moisture• Door management• Strip curtains / air lock• Try dehumidification
– e.g. 8% reduction
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Aux Load Reduction
Double impact / saving• Evaporator fan motors
– E.g. 14% reduction on HT area
• Process fluid pump control
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Leak Reduction
15% leakage doubles power input• Maintenance
– Leak test method– Regime
• Fixed leak detection• Charge monitoring
Tuesday05/02/2002
3 9 15 216 1812Wednesday
06/02/2002
3 9 15 216 1812Thursday
07/02/2002
3 9 15 216 1812Friday
08/02/2002
3 9 15 216 1812
PPM
0
50
100
150
200
250
300
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Control
• Suction and discharge pressure optimisation– ~10% savings
• Defrost on demand– ~2.5% savings
• Floating head pressure– ~ 7% savings
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Re Commissioning
Re set suction & discharge set pointsExample savings:• 21.2% on HT pack• 13.6% saving on LT pack• 6.9% on total site energy
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Maintenance is Vital
Ensure there is a maintenance regime which at least …
…cleans condensers and evaporators…checks for leaks (and repairs them!)…does an eyes & ears system check
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