Refrigeration Plant and Condenser Efficiency Mr Scott Clydsdale WEA Aug. 2012 NZ
REFRIGERATION PLANT AND CONDENSER EFFICIENCY by Scott Clydesdale (B.E. Mech. Hons.)
GORDON BROTHERS INDUSTRIES PTY LTD
WINERY ENGINEERING ASSOCIATION
9 SEPTEMBER 2012
Overview • Refrigeration 101 • Power Usage Figures • Condenser and Compressor Relationship
• Brine / Refrigeration Temperatures Costs • Differing Refrigerants for Various Loads • Optimising Heat Exchanger Surface Area
• Correct Use of Variable Speed Drives • Other energy saving methods…
What is Refrigeration?
• Refrigeration is removing heat from one location and transferring it to another location. (e.g. taking heat from a room and putting it outside.)
• The vapour compression system is the most common form of refrigeration.
• Uses a compressor as the ‘driving force’ • The heat is taken out of the room via the evaporator and then condensation causes this heat to be rejected to the outside environment.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System… 1.
Compression Refrigerant vapour is taken at a dry and saturated suction condition and compressed to some higher pressure and temperature level.
Compressor Types The 2 main types of compressors are…
• Reciprocating compressors
• Screw compressors
• • • •
Reciprocating Compressors These compressors have reciprocating pistons. Suitable for smaller plants. They have good part load Generally require more maintenance than screws
• • •
Screw Compressors Typically used in larger plants Less efficient at part load. Less maintenance but more expensive initially.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System…
2.
Condensation Cools down the high pressure refrigerant from the compressor by rejecting heat to the atmosphere. Process occurs at a constant pressure, and refrigerant enters as a vapour and leaves as a liquid.
Evaporative Condenser Water spray eliminators
Saturated air outlet Gaseous Ammonia
Water sprays
Condensing coils Liquid Ammonia
Centrifugal fans
Water sump Water pump
Other Types of Condenser
Water Cooled (Plate type or shell & tube)
Air Cooled
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System…
3.
Expansion Usually occurs through a valve. As high pressure refrigerant passes through the valve the pressure is lowered effectively lowering the temperature.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System… 4.
Evaporation Low pressure liquid refrigerant enters the evaporator. Heat is transferred to the refrigerant in the evaporator from the warm room / product. The heat causes the liquid refrigerant in the evaporator to boil off into vapour.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Pressure
Basic Refrigeration Cycle
Liquid
Vapour
Liquid+Vapour
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
Liquid+Vapour Compression Evaporation
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
1250kPa +35ºC (R717)
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
1250kPa +35ºC (R717)
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
1250kPa +35ºC (R717)
1066kPa +30ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Power Usage in a Refrigeration Plant
Power Usage in a Winery
Pumping 5% Compressed Air 8%
Lighting 5%
Refrigeration 45%
Winery Equipment 16%
Boilers 21%
*According to AWRI “Identify cost saving with an Energy Audit”
Condenser Use vs Compressor Use High Load Periods
• One third of the energy usage in a winery is by the compressors.
• Only 5% is the condensers… • Makes sense to operate the condensers ‘harder’ to allow the compressors to operate ‘easier’?
• This is valid at periods of high load. (i.e. Summer / early Autumn during crushing).
• BUT…
Condenser Use vs Compressor Use Low Load Periods
• What about late Winter when the compressors are unloaded and the condensers are still running flat out?
• Compressors may be operating at 20% of full load (so now only 7% of winery power usage)…
• Are the condenser still best to run flat out? (And still absorb 5% of the winery loads?)
• Remember fan power is proportional to its speed cubed (so small speed reductions equal big power savings)…
• i.e. 80% fan speed = 0.8³ = 51% power usage • The ambient temperatures are now much lower too, so the condensers are more efficient… Maybe 80% fan speed is more than enough?
Condenser Use vs Compressor Use All Load Periods
• Need a ‘black box’ control system to optimise the condenser performance to ensure maximum capacity from the minimum power usage.
• Needs to take into account ambient conditions as well as plant performance.
• Also needs to use characteristic efficiencies of all devices so part load performance is also taken into account.
Brine / Refrigerant Temperatures
• It costs more to make a system colder! • 2.5% compressor running cost increase per 1°C temp. drop! • What temperature does your winemaker really need? • Does must at 35°C need -10°C brine to get to a target 15°C? • Or would 0°C brine work OK?
Differing Refrigerants / Temperatures for Loads
• Can you split the plant to give two or more refrigerant temperatures for different loads (i.e. must chilling vs fermentation tanks)?
• If the plant remains at a common temperature, can we use direct ammonia on the coldest temperature loads and brine / chilled water on the others?
• (The overall plant temperature is generally set to service the lowest load, even if it is only 20% of the duty overall).
• Saving 5 – 15% energy usage on half the plant is still a big saving
• It costs more to make a system colder, even part of a system!
Heat Exchanger Surface Area
• Larger surface heat exchangers can more efficiently transfer heat
• For a fixed product temperature, a larger heat exchanger surface area will allow a higher refrigerant temperature to be used
• Cost of high surface area increases, but what is power worth for the life of the heat exchanger?
• Remember raising the refrigerant temperature by 5°C can reduce your power bill by 12%!!
Correct Use of Variable Speed Drives
• Is it beneficial to have a VSD on a motor that operates at close to 100% most of the time?
• No!! • VSDs are not 100% efficient (like a contactor) so should only be installed on drives with part load performance requirements.
• Usual losses in the order of 1 to 3%!! • Use VSD only on trim machines (compressors) and not on the base line machine.
• Use VSD on condenser fan for part load energy savings
Selection of Screw Compressor Cooling Method Screw compressors require some source of external cooling to overcome the heat of compression. The options include…
• Liquid injection where liquid refrigerant is expanded into the •
rotors, cooling the machine… Simple to arrange but this system “steals” around 7% of the machine performance by “taking up” compression space in the rotors.
• Water cooled oil cooler… • Good efficiency but cleaning issues with the water side of the shell and tube heat exchanger.
• Thermosyphon oil cooler… • Good efficiency and zero maintenance, but requires elevation of the liquid receiver to achieve this.
Economising Screw Compressors
• Allows for some gas to be introduced part way through the compression process.
• This partial compression can be used to subcool the liquid feed to various loads, improving the overall system efficiency
Heat Reclaim from the Refrigeration Plant
• “Waste” heat can be reclaimed from a refrigeration plant in several ways…
• Desuperheating with a heat exchanger installed upstream or in parallel with the condenser to capture the superheat of the refrigerant.
• Can cost more energy than can be reclaimed!
Desuperheater Location
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER
Desuperheater COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Pressure
Desuperheater Added
Liquid
Vapour
Condenser
Desuperheater 1250kPa +35ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Desuperheater Added
Pressure
Liquid
Vapour 1288kPa +36ºC (R717)
1250kPa +35ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Desuperheating
• Condenser may run ‘unloaded’ for periods that desuperheater is working.
• Still need to size the condenser for periods when hot water is not required (or tank is already hot)
• Need to do holistic analysis for any heat reclaim system.
Summary
Ensure your refrigeration plant is designed for maximum efficiency with…
• Optimisation of condenser control • Highest possible refrigerant temperatures • Splitting plant where varying temperature requirements exist • Maximising heat exchanger surface areas • Correct use of variable speed drives • Not using liquid injection as an oil cooling method on screw compressors
• Holistic analysis of any heat reclaim used.
REFRIGERATION PLANT AND CONDENSER EFFICIENCY by Scott Clydesdale (B.E. Mech. Hons.)
GORDON BROTHERS INDUSTRIES PTY LTD
WINERY ENGINEERING ASSOCIATION
9 SEPTEMBER 2012
GORDON BROTHERS INDUSTRIES PTY LTD
WINERY ENGINEERING ASSOCIATION
9 SEPTEMBER 2012
Overview • Refrigeration 101 • Power Usage Figures • Condenser and Compressor Relationship
• Brine / Refrigeration Temperatures Costs • Differing Refrigerants for Various Loads • Optimising Heat Exchanger Surface Area
• Correct Use of Variable Speed Drives • Other energy saving methods…
What is Refrigeration?
• Refrigeration is removing heat from one location and transferring it to another location. (e.g. taking heat from a room and putting it outside.)
• The vapour compression system is the most common form of refrigeration.
• Uses a compressor as the ‘driving force’ • The heat is taken out of the room via the evaporator and then condensation causes this heat to be rejected to the outside environment.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System… 1.
Compression Refrigerant vapour is taken at a dry and saturated suction condition and compressed to some higher pressure and temperature level.
Compressor Types The 2 main types of compressors are…
• Reciprocating compressors
• Screw compressors
• • • •
Reciprocating Compressors These compressors have reciprocating pistons. Suitable for smaller plants. They have good part load Generally require more maintenance than screws
• • •
Screw Compressors Typically used in larger plants Less efficient at part load. Less maintenance but more expensive initially.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System…
2.
Condensation Cools down the high pressure refrigerant from the compressor by rejecting heat to the atmosphere. Process occurs at a constant pressure, and refrigerant enters as a vapour and leaves as a liquid.
Evaporative Condenser Water spray eliminators
Saturated air outlet Gaseous Ammonia
Water sprays
Condensing coils Liquid Ammonia
Centrifugal fans
Water sump Water pump
Other Types of Condenser
Water Cooled (Plate type or shell & tube)
Air Cooled
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System…
3.
Expansion Usually occurs through a valve. As high pressure refrigerant passes through the valve the pressure is lowered effectively lowering the temperature.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Four Processes in a Refrigeration System… 4.
Evaporation Low pressure liquid refrigerant enters the evaporator. Heat is transferred to the refrigerant in the evaporator from the warm room / product. The heat causes the liquid refrigerant in the evaporator to boil off into vapour.
Basic Refrigeration Cycle
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Pressure
Basic Refrigeration Cycle
Liquid
Vapour
Liquid+Vapour
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
Liquid+Vapour Compression Evaporation
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
1250kPa +35ºC (R717)
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation
Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
Expansion
1250kPa +35ºC (R717)
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Basic Refrigeration Cycle
Pressure
Liquid
Vapour
Condensation
1250kPa +35ºC (R717)
1066kPa +30ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Power Usage in a Refrigeration Plant
Power Usage in a Winery
Pumping 5% Compressed Air 8%
Lighting 5%
Refrigeration 45%
Winery Equipment 16%
Boilers 21%
*According to AWRI “Identify cost saving with an Energy Audit”
Condenser Use vs Compressor Use High Load Periods
• One third of the energy usage in a winery is by the compressors.
• Only 5% is the condensers… • Makes sense to operate the condensers ‘harder’ to allow the compressors to operate ‘easier’?
• This is valid at periods of high load. (i.e. Summer / early Autumn during crushing).
• BUT…
Condenser Use vs Compressor Use Low Load Periods
• What about late Winter when the compressors are unloaded and the condensers are still running flat out?
• Compressors may be operating at 20% of full load (so now only 7% of winery power usage)…
• Are the condenser still best to run flat out? (And still absorb 5% of the winery loads?)
• Remember fan power is proportional to its speed cubed (so small speed reductions equal big power savings)…
• i.e. 80% fan speed = 0.8³ = 51% power usage • The ambient temperatures are now much lower too, so the condensers are more efficient… Maybe 80% fan speed is more than enough?
Condenser Use vs Compressor Use All Load Periods
• Need a ‘black box’ control system to optimise the condenser performance to ensure maximum capacity from the minimum power usage.
• Needs to take into account ambient conditions as well as plant performance.
• Also needs to use characteristic efficiencies of all devices so part load performance is also taken into account.
Brine / Refrigerant Temperatures
• It costs more to make a system colder! • 2.5% compressor running cost increase per 1°C temp. drop! • What temperature does your winemaker really need? • Does must at 35°C need -10°C brine to get to a target 15°C? • Or would 0°C brine work OK?
Differing Refrigerants / Temperatures for Loads
• Can you split the plant to give two or more refrigerant temperatures for different loads (i.e. must chilling vs fermentation tanks)?
• If the plant remains at a common temperature, can we use direct ammonia on the coldest temperature loads and brine / chilled water on the others?
• (The overall plant temperature is generally set to service the lowest load, even if it is only 20% of the duty overall).
• Saving 5 – 15% energy usage on half the plant is still a big saving
• It costs more to make a system colder, even part of a system!
Heat Exchanger Surface Area
• Larger surface heat exchangers can more efficiently transfer heat
• For a fixed product temperature, a larger heat exchanger surface area will allow a higher refrigerant temperature to be used
• Cost of high surface area increases, but what is power worth for the life of the heat exchanger?
• Remember raising the refrigerant temperature by 5°C can reduce your power bill by 12%!!
Correct Use of Variable Speed Drives
• Is it beneficial to have a VSD on a motor that operates at close to 100% most of the time?
• No!! • VSDs are not 100% efficient (like a contactor) so should only be installed on drives with part load performance requirements.
• Usual losses in the order of 1 to 3%!! • Use VSD only on trim machines (compressors) and not on the base line machine.
• Use VSD on condenser fan for part load energy savings
Selection of Screw Compressor Cooling Method Screw compressors require some source of external cooling to overcome the heat of compression. The options include…
• Liquid injection where liquid refrigerant is expanded into the •
rotors, cooling the machine… Simple to arrange but this system “steals” around 7% of the machine performance by “taking up” compression space in the rotors.
• Water cooled oil cooler… • Good efficiency but cleaning issues with the water side of the shell and tube heat exchanger.
• Thermosyphon oil cooler… • Good efficiency and zero maintenance, but requires elevation of the liquid receiver to achieve this.
Economising Screw Compressors
• Allows for some gas to be introduced part way through the compression process.
• This partial compression can be used to subcool the liquid feed to various loads, improving the overall system efficiency
Heat Reclaim from the Refrigeration Plant
• “Waste” heat can be reclaimed from a refrigeration plant in several ways…
• Desuperheating with a heat exchanger installed upstream or in parallel with the condenser to capture the superheat of the refrigerant.
• Can cost more energy than can be reclaimed!
Desuperheater Location
Heat Removed High Pressure Liquid
High Pressure Gas
CONDENSER
Desuperheater COMPRESSOR
EXPANSION EVAPORATOR Low Pressure Liq+Gas
Low Pressure Gas
Heat Added
Pressure
Desuperheater Added
Liquid
Vapour
Condenser
Desuperheater 1250kPa +35ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Desuperheater Added
Pressure
Liquid
Vapour 1288kPa +36ºC (R717)
1250kPa +35ºC (R717)
Expansion
Liquid+Vapour Compression 136kPa -15ºC (R717)
Evaporation Capacity
Power Abs. Enthalpy
Desuperheating
• Condenser may run ‘unloaded’ for periods that desuperheater is working.
• Still need to size the condenser for periods when hot water is not required (or tank is already hot)
• Need to do holistic analysis for any heat reclaim system.
Summary
Ensure your refrigeration plant is designed for maximum efficiency with…
• Optimisation of condenser control • Highest possible refrigerant temperatures • Splitting plant where varying temperature requirements exist • Maximising heat exchanger surface areas • Correct use of variable speed drives • Not using liquid injection as an oil cooling method on screw compressors
• Holistic analysis of any heat reclaim used.
REFRIGERATION PLANT AND CONDENSER EFFICIENCY by Scott Clydesdale (B.E. Mech. Hons.)
GORDON BROTHERS INDUSTRIES PTY LTD
WINERY ENGINEERING ASSOCIATION
9 SEPTEMBER 2012
