UNIT 4: REFRIGERATION PRINCIPLE

UNIT 4: REFRIGERATION PRINCIPLE 4.1 Introduction i. Buildings are essentially enclosed spaces designed for the purpose of accommodating occupants and their activities. ii. Thermal comfort is incorporated with the building design in terms of architectural design (e.g Buildings designed with transparent atriums to allow natural light to emanate buildings) as well as M & E services (Sufficient lighting, air conditioning, etc). Thermal comfort for humans determined by several factors, e.g clothing of occupants, average temperature of locality, etc. Average thermal comfort: 27 Degrees Celsius. Thermal comfort: Crucial in preventing building sickness in occupants (Especially important for office/working environment, as it will affect productivity); iii. Refrigeration cycle: Designed to remove heat from building s premises to external environment, as well as heat up building interior during periods of cold weather. 1

iv. Calculations: a) British Thermal Unit (BTU): Unit of energy to measure power consumption for air con units; 1 BTU = amount of heat necessary to raise one pound of water (0.454 kg) y 1 degree Fahrenheit (F) or 0.56 degrees Centigrade; 1 BTU = 252 calories or 1.055 kilo Joules (kj); 1,000 kwh = 3.41 million BTU; Necessary to design power requirements for cooling; If BTU is sized too high, humidity of cooled area may be insufficiently removed. If BTU is too low, insufficient cooling may occur; Typical methodfor sizing BTU: Area of room (In square feet) x 25; E.g: If room size is 12 feet by 15 feet, area is 180 square feet x 25 = roughly 4500 BTU; May include other factors: E.g Temperature and humidity of the locality, no of occupants (Generally, 1 occupant would represent about 1000 BTU), etc. b) Volumetric Air Flow: Volumetric measurement of airflow per unit time from a ventilation (Natural or mechanical) device; Usually in metre cube per hour (CMH) or Liters/Sec; Used for designing air flow requirements in accordance to Code of Practice for locality/country (SS 553: 2009 for Singapore). Volumetric Flow Rate Conversion Chart 2

4.2 Understanding the Refrigeration Cycle 4.2.1 How the Refrigeration Cycle Works i. Refrigerant cycle functions as a heat exchanger. ii. The medium for heat exchange: a) Chemical refrigerant: Used for split units, VRV systems and other air conditioning systems; Usually chosen for their low boiling points; Very volatile chemicals, hence have huge capacity of heat absorption and dissipation. Because of their volatility, copper piping needs to be insulated by armaflex to reduce the effects of condensation; Usually stored in canisters complete with nozzles for ease of topping up in air con systems; Environmental Issues: Old refrigerant types using CFCs are known to threaten the ozone layer, and are usually banned in most countries (E.g Freon refrigerants manufactured under DuPont). Typical Chemical Refrigerant for Stored in Canister form 3

b) Water: Used in water chillers. Transfers heat from a chemical refrigerant to a heat dissipation element (i.e Cooling Tower). Usually made of tougher materials, e.g stainless steel. Chilled water piping (Stainless steel) in water chiller system. Connection to local water supply helps to replenish losses in system Copper tubing for refrigerant in A/C system c/w armaflex insulation iii. How the Refrigeration Cycle Works: a) Compressor: Receives low pressure refrigerant from evaporator; Compresses and pumps the refrigerant throughout the refrigeration system; Provides sufficient high pressure to drive the refrigerant system; b) Condenser: Receives high pressure refrigerant from compressor; Transfers heat from refrigeration system to outside environment; Low Temp High Temp Low Temp Schematic Diagram of Refrigeration Cycle 4

c) (Thermal) Expansion Valve: Receives high pressure refrigerant from Condenser; Controls refrigerant flow to evaporator; Connected to the sensor bulb, installed at evaporator to monitor temperature; Temperature setting (usually via remote control) will relay message to sensor, which will adjust refrigerant flow accordingly to meet set temperature. d) Evaporator: Receives low pressure refrigerant from evaporator; Usually incorporated with a fan coil unit within the building premises; Receives heat from building s premises, which eventually is sent to the condenser to be expelled to the external environment. Compressor and Condenser unit incorporated in outdoor condensing unit. Sensor Bulb Evaporator Unit Split Unit Air Con FCU 5

v. Condensation Issues for Air Can Evaporator/Fan Coil Units Condensation occurs in fan coil units when the cold surface of the evaporator meets the warm air within the building proper; The situation worsens if the fan coil unit is located near sources of air, e.g near the entrance of the lobby, where the constant opening and closing of door causes constant warm air to come into contact with the evaporator; Drain pan is usually located beneath the evaporator coil; Condensate is collected at the pan, which is connected to PVC condensate piping (See picture on right); Condensate piping is usually connected to a drain or waste trap and flows away as waste water in the sanitary system; Regular maintenance is required to prevent backflow of condensate water due to chokage from debris, such as dirt particles. Typical design for condensate piping. Condensate piping is usually designed with a downward gradient, i.e the highest point from the condensate pan must be directed downwards via piping towards the drainage point. Concept similar to sanitary drainage system; Should the condensation piping follow a irregular travel pattern, i.e the piping travels upwards and downwards before reaching the drainage point, the water may not be able to flow to the drainage point. This will cause water to floor backwards to the pan, causing leakage at the FCU; In this case, a condensate pump needs to be installed to pump away the condensate. Typical drainage pump installed to pump away condensate to higher gradient Typical condensate pump 6

vi. Compressors Compressors: Device which increases pressure and decreases volume of a fluid (Hence compressing the fluid) to drive it through a piping system; Electrically driven: Needs a constant power source to drive it; Difference from pumps: Pumps do not compress fluids as their main function is to transport fluids via pressurization (e.g condensate pump used to transport a/c condensate to drainage). Compressor Categories Single Stage Centrifugal Compressor Types of A/C Compressors: a) Reciprocating Compressors Range: Up to 180kW; Typical for small localized systems, e.g window sill A/C units; Positive displacement type; How it Works Piston moves downwards, creating vacuum in cylinder space above the piston. This draws refrigerant into the space from the intake valve (Outlet valve is in closed position during this process); When piston reaches bottom, it springs up and compresses the refrigerant. Compressed refrigerant is forced into the exhaust valve. At this point, the intake valve is closed; Cycle is repeated. 7

b) Rotary Screw Compressors Mid-Range: Up to 2MW; Suitable for mid-range chiller systems; Compact, smooth running, limited vibration; Spring suspensions not required; Consists of Dry Type and Oil Flooded Type. Dry Type: Two rotors (1 is driver, the other is driven) or helical screws used to compress gas; Timing gears are used to ensure rotors maintain precise alignment; Oil-free, suitable for sensitive environments, e.g Hospitals. Helical Screws for compressing Fluids Dry Type Rotary Compressor Oil-flooded type: Similar design, Oil is injected into compression cavities for: Sealing purposes to prevent backflow of refrigerant; Cooling sink for refrigerant discharge; Transferring mechanical energy between driver and driven rotors; Refrigerant gas enters compressor, mixes with oil and exits compressor; Oil is mixed with refrigerant and separated in gas-oil separator before compressed gas is discharged. Oil-flooded Rotary Compressor System Separated oil is returned to micro oil filter for filtering and then to oil cooler to be cooled. 8

c) Rotary Vane Compressors Rotary Motion: Utilizing a series of vanes and slots in the rotor; Rotor is actually offset from center of the housing; How it Works: Rotating shaft rotates and shifts, turning the rotor as well; Vanes move in and out of the slots, causing refrigerant from suction port to be compressed before it leaves the compressor. Cross Section of Rotary Vane Compressor Schematic of Rotary Vane Compressor d) Centrifugal Compressors High Range: 3.5MW; Incorporated with large, centralized chiller systems; How it Works: Rotating disk/impeller rotates in housing, driving refrigerant to force it to the impeller rim; Velocity increases, converts to high pressure and is diverted to condenser. Centrifugal Compressor incorporated in Centralized Air Conditioning System 9

4.3 Air Conditioning & Mechanical Ventilation Types Various types of air conditioning and mechanical ventilation systems are available in the market; While functions may differ in accordance to their allocated functions (e.g Type of refrigeration, Centralized and localized cooling, etc), the basic design is similar to the vapor compression system: Heat absorption (Evaporator), temperature control apparatus (Expansion Valve), heat dissipation (Condenser), propulsion of heat transfer medium (Compressor). 4.3.1 Localized Cooling Systems Designed for localized cooling in small rooms or areas, e.g rooms, offices, etc; Cooling element (Evaporator) is usually incorporated with mechanical fan/blower; Types of localized cooling systems include: a) Window sill air conditioners b) Split systems c) Multi split systems d) Variable refrigerant flow split systems. Window Sill Air Con Installation at Residential Unit 10

i. Window Sill A/C: As the name suggests, the apparatus is mounted on window sill; Follows the vapor compression/refrigerant cycle method; All components inside cabinet unit; Evaporator facing unit interior, condenser facing outside environment; Evaporator: Warm air is drawn from room into filter, then passes back to room as cold air; Condenser: Expels heat from building via blower. ii. Single Split Unit A/Cs Similar to Window Sill A/C; Evaporator and Expansion Valve (Fan Coil Unit, or FCU) housed separately from Condenser Unit (CU: Combination of compressor and Condenser); Comes with Ducting and Ductless units. Ductless Split Unit A/C Ducted Split Unit A/C 11

iii. Multi-Split Unit Air Conditioning Similar to single split unit A/Cs; One CU unit is linked to more than 1 no of FCU units; Advantages: Saves space, without the need to free up floor space to install more CU units; Energy Saving Features. Comparison between single split units and multiple split units iv. Variable refrigerant flow (VRF) Split Unit Systems Also known as VRV (Variable Refrigerant Volume) for Daikin prototypes; Similar to split units, with up to 40 FCUs connected to external modular CU unit; Each FCU is controlled by its own solenoid valve kit, able to regulate refrigerant flow (and temp)individually for each FCU; 12

Prototypes may include heating and cooling elements; Advantages: a) Does not need huge floor space for installation, unlike chiller systems; b) Flexibility of use, as temperature for various FCUs can be adjusted individual unlike centralized systems; c) Energy saving properties; d) Can be used for a variety of settings, e.g Industrial premises, retail, offices, etc. 4.3.2 Reverse Cycle Heat Pump (HVAC) i. In countries where the 4 seasons are prevalent, HVAC (Heating Ventilation and Air Conditioning) systems are required to provide both cooling and heating functions within buildings. ii. Reverse Cycle Heat Pumps can be employed to serve a HVAC function: System is similar to vapor compression cycle; Uses refrigerant similar to standard split unit A/C; Can be used for cooling and heating purposes by reversing refrigerant cycle flow; Requires a heat sink to transfer heat (Similar to CU unit of Vapor compression cycle): Outside environment; Ground. Outdoor Units for Heat Pumps 13

iii. How it works: a) For cooling purposes: Outdoor unit serves as condenser, expelling heat via fan (Similar to split unit a/c); Internal unit serves as a evaporator, receiving heat and directing it to the outdoor unit; b) For heating purposes: Outdoor unit serves as evaporator, as heat is received by the outdoor unit and directed to the indoor unit via the heated refrigerant; Internal unit serves as a condenser, receiving heat and directing it to the indoor unit to heat the building interior. 14

iv. Geothermal Heat Pump Similar to normal heat pumps, but uses ground as heat sink; During winter, heat is pumped from the ground and used to heat building; During summer, the reverse happens, and the building is cooled. Advantages: Cheap: Electricity is required only to drive the compressor; Environmentally friendly: Significantly low CO2 emissions; Highly efficient, as it utilizes the ground as the heat sink (Especially so for heating purposes). 4.3.3 Absorption Cooling i. Using heat (Derived from a heating element, e.g gasoline, solar power, etc) to drive heat exchange medium (instead of compressor in typical vapor compression cycle) as a driving force; ii. Can be localized or centralized A/C systems; iii. Preferable to vapor compression types in terms of: Lower operating costs (Because no compressor is used); Unreliable electricity supply in locality; Where there is a supply of surplus heat source (e.g economizer from a nearby boiler can use recycled heat to be used for absorption cooling); Can utilize solar power and other forms of clean energy to provide the heating element. A localized domestic Absorption Cooling Refrigerator 15

iv. Basic Absorption Cooling Cycle a) Heat is the driving force for the absorption cooling cycle; b) Evaporation Process: Liquid refrigerant enters evaporator; Absorbs heat from evaporator s surroundings, becomes gaseous form; c) Absorption Process: Gaseous refrigerant enters absorber; Low pressure gaseous refrigerant dissolved in another fluid; This allows for more refrigerant to evaporate in evaporator due to reduced partial pressure; d) Regeneration Process: Dissolved refrigerant in absorber heated, allowing refrigerant to evaporate (Similar to compressor function); Evaporated refrigerant condenses via heat exchanger, replenishes refrigerant in evaporator (Similar to condenser function). v. Single Pressure Absorption Cycle Capacity range: 5kW up to 12MW; Utilizes absorbent (Lithiumbromide) and refrigerant (Water). Heat source: Steam, hot water; Coefficient of Performance (COP): <1; How it Works: a) Generator: Heat source heats up the lithium-bromide solution; Water evaporates and enters condenser; Concentrated lithium-bromide solution flows into absorber, to be pumped back to generator after being cooled by cooling water; 16

b) Condenser: Receives vaporized water (Refrigerant) from generator; Cooling water system: Transfers heat from vaporized water to cooling tower for external cooling; Vaporized water condenses, pressure increases, temperature decreases and flows to evaporator; c) Evaporator: Specialized nozzles spray the low temperature refrigerant on chilled water piping system circulating between the indoor cooling unit (Usually an air handling unit) and the evaporator; Cooled chilled water circulated back to indoor unit; Heated refrigerant is absorbed into solution in absorber. d) Absorber Continuous water-lithium bromide solution constantly receives heated water vapor from evaporator, as well as lithium bromide solution from generator; Solution is pumped back to generator to complete cycle via solution pump. vi. Two-Stage/Double Effect Absorption Chillers More efficient than single stage/single absorption cycles/units; Requires higher temperature from heat source at generator (>140 degrees Centigrade); Uses 2 generators: a) Main generator (steam fed or direct gas fired burners): Generates heat for the system; b) Low temperature generator: Recovers heat which would otherwise be lost to cooling tower to drive the refrigerant. c) Triple effect absorption chillers also available as well, similar principle as two stage absorption chillers. 17

vii. Water Spray Absorption Refrigeration Inclusive of dehumidifying function; How it works: 1) Warm, moist air enters ducting system fan air inlet; Salt solution sprayed; Humidity reduced; Temperature relatively unaffected; 2) Warm, dry air enters evaporative cooler; Pure water sprayed; Temperature reduced; Humidity increases; 3) Cool, moist air enters fan air outlet; Cool moist air is sprayed with salt solution at fan air outlet to reduce humidity; 4) Cool dry air is exits the system. Refrigeration system (In Blue): Salt solution tank collects spray from both pure water and salt solution, where it is heated, condensed and pure water is collected to complete refrigerant cycle. 1 2 3 4 18

4.3.4 Chilled Water Air Conditioning Systems i. Designed for centralized cooling over large properties, e.g shopping malls; ii. How it works: a) Condensing Water System: Uses water as refrigerant; Transfers heat from compressor to cooling tower; Heat is dissipated from cooling tower; Condensing pump: Provides energy for water to transverse system; b) Chiller: Acts as a heat exchanger between chilled water system and condenser water system; Vapor compression cycle; Chemical refrigerant used; Typical Chilled Water A/C System incorporating Air Handling Unit System and Exhaust System c) Chilled water system: Uses water as refrigerant; Transfers heat from air handling unit to chiller; Chilled water pump: Provides energy for water to transverse system; d) Air handling unit: Acts as a evaporator for centralized cooling; Ducting air vents split to various locations throughout designated locations for cooling purposes; Cooling fans installed to prove mechanical ventilation; Filters installed to filter out dust from outlets; Dehumidifier installed to reduce humidity; e) Exhaust system: Mechanical ventilation system designed to expel stale air from building proper. Typical Air Handling Unit 19

Tutorial Questions 1) Explain in detail, with the aid of illustration, how the Refrigeration Cycle (Vapor Compression Cycle) works (20 marks). 2) Condensation issues in Fan Coil Units (FCUs) are a common problem in many air con installations, causing FCU units to leak, or sweat. i. Briefly describe how condensation occurs (2 points). ii. Explain, with the aid of a simple sketch, how condensation piping should be installed so as to prevent leakage in air cons (10 points). iii. Explain, with the aid of a simple sketch, the condition(s) whereby air con condensation pumps should be installed in condensation piping system (8 points). 3) Illustrate with the aid of a simple sketch, on how the Absorption Cooling Cycle works (20 marks). 4) Heat Ventilation Air Conditioning (HVAC) systems are common in some countries. i. Identify the type of environmental conditions which will necessitate the use of HVACs (2 points); ii. Identify the basic functions of the HVAC systems (2 points); iii. Describe in detail how Reverse Cycle Heat Pump works(9 points). iv. Briefly describe how the Geothermal Heat Pump works, and list down the advantages of using the geothermal heat pump (7 points). 20