HVAC Fundamentals & Refrigeration Cycle
Change of State of Water & the Refrigeration Cycle Change of State Water The Basic Refrigeration Cycle Types of DX Systems The Chilled water System The Cooling Tower
Change of State - Water Aggregate state changes Solid to Liquid Liquid to Solid (fusion) Liquid to gas (vaporisation) Gas to liquid (liquefaction) : Melting : Solidification : Evaporation : Condensation
Change of State - Water Melting / Freezing 1 kg 335 Kj of heat are required to melt 1 kg of ice at 0 C. The subsequent heat of melting in the water is thus referred to as (imperceptible) latent heat Latent heat of ice 0 C 0 C 335 kj
Fig. 06 Change of State - Water Landis Heating At standard pressure (atmospheric pressure at sea level of 1.013 bar), a sensible quantity of heat equal to 419kj must be added to 1 kg of water to raise its temperature from 0 C to 100 C. At this point water boils and vaporisation commences. Log p (bar) 100 10 1 0,1 0,01 0,001-100 0 100 200 300 400 C Boiling point of water is dependent on air pressure Siemens Building Technologies Einführung in die HLK-/Haustechni Modul_01_K04 Physikalische Gru Sales and Application Training 10.9
Change of State - Water Vaporisation 2257 kj of heat is required to convert 1kg of water at 100 C to steam at 100 C This is the latent heat of vaporisation. If we add the 419kj of sensible heat used to bring the water to 100 C (from 0 C), the total heat, ENTHALPY, of steam at 100 C is 2676 kj 100 C 1 kg 2676 kj 419 kj 2257 kj
Change of State - Water Superheating 2676 kj If heat is added to saturated steam at 100 C, the temperature further increases. This is also sensible heat. To increase the temp of the kg of steam from by 15 C (say from 100 to 115 C), the heat required is 28.3 KJ. Q = m.cp.(t1-t2) =1x 1.88 x (115-100) = 28.3 kj 1 kg 100 C 100 C 2704,3 kj + 28,3 kj
Change of State - Water Condensation 2 Process of vapour (steam in this case) giving up its heat to convert itself to liquid (in this case water) at the same temperature In our example this value is 2257kj Loss free processes are not possible in practice 1 2257 kj 3 1 - saturated steam, (1kg),100 C, 2 - cooling water (cold), 3 - cooling water (warm, +2257kj) 4 - condensate (1 kg water), 100 C 4
Change of State - Water C 115 100 B C D -335 419 2257 28,3 10 0 A Temp-Enthalpy diagram 0 419 2676 2704,3 h (kj / kg) A-B : Heating the liquid (sensible heat) B-C : Vaporisation (Latent heat) C-D : Superheating (sensible heat)
Sensible / Latent Heat Sensible heat refers to the heat exchanged/transferred that causes a change in temperature of matter Latent heat refers to the heat exchanged/transferred that causes a change in the state (or phase) of matter
Decreased Temperature at Saturated pressure A change in Sensible Heat P T sat T Liquid Subcooled or Compressed Liquid
Phase Changes A change in Sensible Heat When P=P(sat.) and T=T(sat), any heat transfer will cause a phase change. Saturated Liquid: Fluid is 100% liquid P Mixture: Fluid is part liquid part vapor with some quality x T=T sat x represents the ratio of vapor to liquid present in the mixture. Fluid Saturated Vapor: Fluid is 100% vapor
Increased Temperature at Saturated pressure A change in Sensible Heat T P T sat Vapor Superheated Vapor
Boiling Substance changes from a liquid to a vapor A change in LATENT Heat. Substance evaporates or boils at its saturation temperature at given pressure
Condensation Condensate Water Vapor Change in phase from a vapor to a liquid Water vapor suspended in the surrounding air condenses when it comes in contact with the the surface (of the glass) which is at a temperature lower than the saturation temperature (dewpoint) of the surrounding atmosphere.
Refrigeration Refrigeration is... Cooling by the Removal of HEAT.
Refrigeration Cycle Vapour Compression System Log p 1 3,1 bar A 1 - heat transfer unit - evaporator Vaporisation (R134a) B h [kj / kg] A-B Vaporisation
Refrigeration Cycle Vapour Compression System Log p h 12 bar C 1 2 3,1 bar A 1 - heat transfer unit - evaporator B h [kj / kg] B-C Compression A-B Vaporisation
Refrigeration Cycle Vapour Compression System Log p 3 D 12 bar C 3 - heat transfer unit - Condenser 1 2 3,1 bar A 1 - heat transfer unit - evaporator B h [kj / kg] C-D Condensation B-C Compression A-B Vaporisation
Refrigeration Cycle Vapour Compression System Log p 3 D 12 bar C 3 - heat transfer unit - Condenser ADIABATIC EXPANSION PROCESS 4 1 2 3,1 bar A A 1 1 - heat transfer unit - evaporator B D-A1 Expansion C-D Condensation B-C Compression A-B Vaporisation h [kj / kg]
Refrigeration Cycle Coefficient of Performance Refrigerating Effect : The heat absorbed by the evaporation process Kj/Kg Pressure Evaporator Refrigerant absorbs heat from load Enthalpy
Refrigeration Cycle Coefficient of Performance Refrigerating Effect : The heat absorbed by the evaporation process Kj/Kg Pressure Adding a subcooler Refrigeration Effect Increases Refrigerating Effect Increases energy efficiency Enthalpy
Refrigeration Cycle Coefficient of Performance Pressure Condenser Refrigerant rejects heat to atmosphere Metering Device Evaporator Compressor Refrigerant absorbs heat from load Enthalpy
Refrigeration Cycle Coefficient of Performance Pressure Compressor Head Pressure Enthalpy
Head Pressure Refrigeration Cycle Coefficient of Performance Pressure Condenser Lowers head pressure Evaporator Enthalpy
Head Pressure Refrigeration Cycle Coefficient of Performance Pressure Lowering condensing temperature Condenser Lowers head pressure Evaporator Reduces compressor work Enthalpy Reduces Energy Consumption
Head Pressure Refrigeration Cycle Coefficient of Performance Pressure Condenser Coefficient of Performance : Ratio of the Refrigerating Effect to the Work done by the compressor Evaporator Enthalpy Work done by the compressor
Refrigeration Cycle Coefficient of Performance Pressure D 3 C Energy absorbed from chilled water Compression power input to refrigerant Energy released to cooling water / atmospheric air + = A 1 B 2 Coefficient Of Performance(COP) Refrigeration Capacity = Power Consumed by Compressor h1 Enthalpy h2 h3 = 1 2 = h2-h1 h3-h2
Refrigeration Cycle Vapour Compression The Four Factors Affecting Refrigeration Efficiency Evaporating temperature/pressure / Condensing temperature/pressure Subcool / Superheati
Factors Affecting Refrigeration Efficiency Evaporating temperature/pressure Condensing temperature/pressure Subcool Superheat
Refrigeration Cycle Analysis 1, Effect of Evaporating Temperature on Chiller Efficiency When evaporating temperature drops, refrigeration cycle changes from 1-2-3-4 to 1-2 -3-4: Pressure 4 1 2 3 3 Refrigeration effect decreases from 2-1 to 2-1 Compression power increases from 3-2 to 3-2 COP decreases. 1 2 Enthalpy
Refrigeration Cycle Analysis 2, Effect of Condensing Temperature on Chiller Efficiency When condensing temperature increases, refrigeration cycle changes from 1-2-3-4 to 1-2 -3-4: Pressure 4 4 3 3 Refrigeration effect decreases from 2-1 to 2-1 Compression power increases from 3-2 to 3-2 COP decreases 1 1 2 Enthalpy
Refrigeration Cycle Analysis 3, Influence of Sub-cooling on Chiller Efficiency Subcool=saturation temp - temp of refrigerant leaving condenser Pressure No Sub-Cooling Metering Device Condenser Compressor Evaporator Enthalpy
Refrigeration Cycle Analysis 3, Influence of Sub-cooling on Chiller Efficiency Subcool=saturation temp - temp of refrigerant leaving condenser Pressure 2 Sub-cooling 1 Subcool( )= T1-T2 Refrigeration effect increases : B-A > B -A Compression power keeps the same COP increases A A B Enthalpy
Refrigeration Cycle Analysis 4, Influence of Superheat on Chiller Efficiency Why we need superheat- To prevent liquid refrigerant from entering compressor Pressure Pressure Saturation Vapor Superheated Vapor Enthalpy Enthalpy Superheat
Refrigeration Cycle Analysis 4, Influence of Superheat on Chiller Efficiency Effect of superheat on refrigeration efficiency: Pressure Ineffective superheat: occur on the suction pipe. When ineffective superheat occurs: Superheated Vapor Superheat Enthalpy Compression power increases Refrigeration effect keeps the same COP decreases
The Direct Expansion Unit Direct Expansion Unit Refrigerant Directly Expands, Vaporizes and Causes Cooling Window Air Conditioner Split Air Conditioner Ducted Split Air Conditioner Packaged Air Conditioner Free Standing (Cabinet Type) Packaged Air Conditioner Ducted Type
Summary HVAC Fundamentals / Refrigeration Cycle Addition of heat energy will increase the temperature of a substance by increasing its molecular activity Heat added which results in a perceptible temperature change (and that can be measured directly with a thermometer is called Sensible Heat) Heat added which results in the complete transformation of a substance from its present state to another state (ex solid to liquid / liquid to solid, liquid to gas / gas to liquid etc.,) is called Latent Heat. Boiling point of a substance can be varied by altering the pressure acting over it. Lower the pressure, faster is the boiling point. In a Direct Expansion System, liquid refrigerant (whose boiling point is low at standard atmospheric pressures), absorbs the heat of its surrounding and vaporizes, thereby cooling the surroundings from where it absorbs the heat.
Summary HVAC Fundamentals / Refrigeration Cycle The refrigerant cycle consists of the 4 basic processes, evaporation, compression, condensation and expansion The DX System, (Direct Expansion System) has several configurations such as Window Air Conditioner Split Air Conditioner Ducted Split Air Conditioner Packaged Air Conditioner Free Standing (Cabinet Type) Packaged Air Conditioner Ducted Type