. Air Conditioning Inspections for Buildings Efficiency of Air Conditioning Systems PRESENTED BY NIRAJ MISTRY aircon@stroma.com
Cooling Load The Refrigeration Load Cooling load on refrigeration system determines: Plant size Power consumption This does not effect the COP. No refrigeration load = no COP or efficiency The smaller the load, the lower the power consumption.
COSP Efficiency of a Refrigeration Cycle The coefficient of system performance (COSP) is expressed as the capacity (kw) divided by the power input (kw) required to produce the cooling: COSP = capacity (kw) power (kw) This takes into account all fans, motors and controls associated with the system. The Seasonal Energy Efficiency Ratio (SEER) is now commonly used as it takes into account the seasonal performance.
COP Efficiency of a Refrigeration Cycle The coefficient of performance (COP c ) is expressed as the useful cooling duty (kw) divided by the power input (kw) required to produce the cooling: HeatOutput HeatingCoP(%) x100 PowerInput The maximum efficiency, Carnot cycle coefficient of performance, is based upon a theoretical thermodynamic cycle on which the actual cycle is based: COP c = T e /(T c -T e ) Where: T c is the condensing temperature (K) T e is the evaporating temperature (K) c denotes a cooling COP This does not take into account all fans, motors and controls associated with the system, compressor only.
Performance Efficiency of a Refrigeration Cycle An actual COP would be typically 50% less than the theoretical COP due to: Deviations from the theoretical cycle Inefficiencies within the practical cycle: Pressure losses Heat transfers Example: A COP of 3.7 means The refrigeration unit will produce 3.7kW of cooling per kw of electrical consumption by the compressor motor. Question: Why is the efficiency greater than 1? Surely this is against the laws of thermodynamics?
Performance Efficiency of a Refrigeration Cycle Answer: Cooling duty is in kw of heat exchange energy, whereas the power input is electrical or work energy. If: power input was expressed as heat energy (i.e. considering the energy used to produce the electricity), then the ratio would be less than 1 (or 100%). COP provides useful information about the running costs of the refrigeration system with respect to the cooling duty.
Performance Efficiency of a Refrigeration Cycle COP is most dependant on temperature difference (temperature lift) between the condensing and evaporating temperatures. Smaller the difference, greater the COP Bigger the difference, lower the COP 2-4% increase in performance for: 1K increase in evaporating temperature 1K decrease in condensing temperature COP is also affected by: Refrigerant type Equipment used Controls and maintenance
COP Factors Lowering the Condensing Temperature Use a condenser with a high basic rating (usually a larger condenser). Allow the condensing temperature to float down with the ambient temperature. Average UK temp is ~10 C, should be used to advantage, as opposed to holding a condensing temperature artificially high. Can save in excess of 25% energy. Use water instead of air as condenser medium. Ensure condensers do not become blocked, or flow of cooling air or water becomes impeded in any other way.
COP Factors Raising the Evaporating Temperature An evaporator with a higher basic rating is used (usually a larger evaporator). The evaporator is defrosted if necessary. Ensure the evaporator is clean and free of blockage.
COP Factors Compressor Efficiency Varies with type and manufacturer. Most efficient compressor for an application should be selected. Depends on: Size of cooling load. Refrigerant used. Temperature of application. Average temperature of the cooling medium (air/water)
COP Factors Amount of Refrigerant This has a significant effect on the temperature lift. Too much or too little reduces efficiency. Systems that leak refrigerant consume more power than necessary. Costs UK refrigerant plant owners and extra 11%. Over charged systems have more refrigerant to lose in the event of a leak environmentally detrimental.
COP Factors Refrigerant Type Variation of this can effect COP up to 10%. Hardware needs to be optimised to the refrigerant for benefit. The most efficient refrigerant for an application depends upon: Compressor type Temperature of the application Average temperature of the cooling medium (air/water).
COP Factors Superheat of the Suction Vapour This needs to be as low as possible. Warmer vapour reduces the capacity of the compressor; Does not reduce its power input. On direct expansion systems achieved by: Correctly controlling the expansion device; Insulating the suction line.
COP Factors Amount of Subcooling This should be as high as possible. Increases the capacity of the system. Does not increase its power input. Liquid line should not: Be insulated. Pass through any hot areas (kitchens, direct sunlight).
Comparison Cooling method Usage in building services Efficiency Capital cost Carbon footprint Vapour compression High High Low Medium Absorption: Generated heat Low Low High High Waste/free energy Low High High Low Air cycle Low Low High Medium Evaporative Medium High Low Low Dessicant Medium High Low Low Carbon dioxide Low Medium High Low
NDHCVCG Non-Domestic Heating, Cooling and Ventilation Compliance Guide (Second Tier Ref: ADL2B) For Cooling Energy Efficiency Ratio (EER) For Chillers; EER is defined as the ratio of cooling energy delivered, divided by the energy input to the cooling plant. For Packaged Air Conditioners; EER is defined as the ratio of energy removed from air within conditioned space, divided by the effective energy input to the unit
Cooling Seasonal Energy Efficiency Ratio (SEER)
Cooling
Cooling Cooling Controls
Cooling Determining The SEER - no part load data
Cooling Determining The SEER by part load data
Ventilation Air Distribution Systems Specific Fan Power (SFP) Relates to the total power (circuit-watts) used by the fan or air handling plant per unit of design flow rate provided. (Units -Watts/litre/second) Limiting values of specific fan power are set out dependant upon system type Limits for existing buildings required for Part L2B Compliance
Ventilation
Pipework
Ductwork
Reference Material Heating, Ventilation, Air Conditioning and Refrigeration, CIBSE Guide B, Chartered Institute of Building Services Engineers, 2005 CIBSE KS13: Refrigeration, CIBSE Knowledge Series, Chartered Institute of Building Services Engineers, 2008 ASHRAE Handbook: Fundamentals, American Society of Heating, Refrigeration and Air Conditioning Engineers, 2001 BS EN 378: Specification for Refrigeration Systems and Heat Pumps; Part 1: 2000: Basic Requirements, Definitions, Classification and Selection Criteria; Part 2: 2000: Design, Construction, Testing, Marking, and Documentation; Part 3: 2000: Installation Site and Personal Protection; Part 4: 2000: Operation, Maintenance, Repair and Recovery, London: British Standard Institution, 2000 Non-Domestic Heating, Cooling, and Ventilation Compliance Guide, Department For communities and local Government Building Regulations Approved Document L2B, Department For communities and local Government