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ISAT 413 - Module III: Building Energy Efficiency

Topic 3: Refrigeration, Heat Pumps and Air Conditioning

Evaporating Cooling

Vapor-Compression Refrigeration

Vapor-Absorption Refrigeration

Other Refrigeration Systems

The Heat Pump

Air Conditioning

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Evaporative Cooling (Eastop, Example 4.8)

In a cooling tower the water to be cooled enters near the top of the tower and flows down through packing in counter flow to an air stream as shown diagrammatically in Fig. 4.34. Air is induced from the base of the tower by a fan mounted on the top of the tower. Using the data given below and neglecting heat losses and pressure losses, calculate:

(i) the required mass flow rate of air induced by the fan for the given cooling duty;

(ii) the mass flow rate of make-up water required.

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Cooling Tower (Evaporative Cooling)

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Data:

Water: Mass flow rate, 15 kg/s; temperature of water at entry, 27oC; temperature of water at exit, 21oC; mean specific heat, 4.18 kJ/kg.K.

Air: Ambient conditions, 1,01325 bar, 23oC dry bulb (DB), 17oC wet bulb (WB); air at exit, saturated with water vapor at 25oC; mean specific heat of dry air, 1.005 kJ/kg.K; fan air power input, 5 kW.

Assumptions: For air at standard atmospheric pressure it can be assumed that the vapor pressure is given by:

For water vapor at low vapor pressure it is a good approximation to assume that the enthalpy is equal to the saturated value at the same dry bulb temperature.

Ktt.barPbarp WBDBgWBs 4107486

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(i) the required mass flow rate of air induced by the fan for the given cooling duty

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Vapor-Compression Refrigeration

P-h Diagram of an Ideal Vapor-Compression Refrigeration CycleP-h Diagram of an Ideal Vapor-Compression Refrigeration Cycle

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Schematic and T-s Diagram for Ideal Vapor-Compression Refrigeration CycleSchematic and T-s Diagram for Ideal Vapor-Compression Refrigeration Cycle

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Schematic and T-s Diagram for Actual Vapor-Compression Refrigeration CycleSchematic and T-s Diagram for Actual Vapor-Compression Refrigeration Cycle

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The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as

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The standard of comparison for refrigeration cycles is the reversed Carnot cycle. A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump, and their COPs are

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The transfer of heat from lower temperature regions to higher temperature ones is called refrigeration. Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles. The working fluids used in refrigerators are called refrigerants. Refrigerators used for the purpose of heating a space by transferring heat from a cooler medium are called heat pumps.

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The most widely used refrigeration cycle is the vapor-compression refrigeration cycle. In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapor and is cooled to the saturated liquid state in the condenser. It is then throttled to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated space.

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Very low temperatures can be achieved by operating two or more vapor-compression Systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading.

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Another way of improving the performance of a vapor-compression refrigeration system is by using multistage compression with regenerative cooling. A refrigerator with a single compressor can provide refrigeration at several temperatures by throttling the refrigerant in stages. The vapor-compression refrigeration cycle can also be used to liquefy gases after some modifications

Q: Cycles? Components? Working Fluids?

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The designation of CFCs (ChloroFluoroCarbons)

is by a two or three digit number:

• The digit on the right represents the number of fluorine (F) atoms;

• The middle digit represents one more than the number of hydrogen (H) atoms;

• The digit on the left represents one less than the number of carbon (C) atoms and omitted when there is only one carbon atom.

Working Fluids – Refrigerants

CHClF2 (R22) or called HCFC 22

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(Notice that researches are now underway to develop acceptable alternatives.)

CCl3F (R11), CCl2F2 (R12), and CHClF2 (R22), HFC 134a,

CCl2FCClF2 (R113) and CClF2CClF2 (R114)

R502 which is an azeotropic mixture (acts as a single substance and cannot be separated into its components by distillation.) of R22 and R115

The commonly accepted designation for refrigerants other than halocarbons (compounds containing carbon, fluorine and chlorine in varying proportions, known as CFCs) is to put the digit 7 in front of the relative molecular mass (e.g. NH3 (R717), CO2 ( R744).)

Some Common CFC, HCFC Refrigerants:

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Vapor-Absorption Refrigeration

Ammonia Absorption Refrigeration CycleAmmonia Absorption Refrigeration Cycle

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Another form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 2000C is absorption refrigeration, where the refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as

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The maximum COP an absorption refrigeration system can have is determined by assuming totally reversible conditions, which yields

where T0, TL, and Ts are the absolute temperatures of the environment, refrigerated space, and heat source, respectively.

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Other Refrigeration Systems

Schematic of Simple Thermoelectric Power Generator

Schematic of Simple Thermoelectric Power Generator

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A Thermoelectric RefrigeratorA Thermoelectric Refrigerator

(Fig. 10-28)

10-13

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A refrigeration effect can also be achieved without using any moving parts by simply passing a small current through a closed circuit made up of two dissimilar materials. This effect is called the Peltier effect, and a refrigerator that works on this principle is called a thermoelectric refrigerator.

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Refrigerator and Heat Pump ObjectivesRefrigerator and Heat Pump Objectives

The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.

The Heat Pump

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Heat Pump Heats a House in Winter and Cools it in Summer

Heat Pump Heats a House in Winter and Cools it in Summer

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Air Conditioning (Eastop, Example 4.11)

An air conditioning plant is designed to maintain a room at 20oC, percentage saturation 50%, which an air supply to the room of 1.8 kg/s at 14oC, percentage saturation 60%. The design outside air conditions are 27oC,percentage saturation 70%. The plant consists of a mixing chamber for re-circulated and fresh air, a cooling coil supplied with chilled water, heating coil, and a supply fan. The ratio of re-circulated air to fresh air is 3; the cooling coil has an apparatus dew point of 5oC, and the refrigeration unit supplying the chilled water has an overall coefficient of performance 2. Neglecting all losses and fan and pump work, calculate:

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(i) the total air conditioning load for the room;

(ii) the required total energy input;

(iii) the required energy input if the energy to the heating coil is supplied from the refrigeration plant condenser cooling water.

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kW.kg

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room on load Total

(i) the total air conditioning load for the room;

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(ii) the required total energy input;

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(iii) the required energy input if the energy to the heating coil is supplied from the refrigeration plant condenser cooling water.

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