Chapter 15 Refrigeration Refrigeration systems: To cool a refrigerated space or to maintain the...

Chapter 15 Refrigeration

 Refrigeration systems:

To cool a refrigerated space or to maintain the temperature of a space below that of the surroundings .

Heat pump systems:

To maintain the temperature within a dwelling or other buildings above that of the surroundings or to provide heat for industrial processes that occur at an elevated temperature.

The knowledge of refrigeration systems would help HVAC engineers in selection of the equipment and fitting it properly into overall system, defining practices consistent with safety and safety standards of the industry, and restrictive regulations on refrigerant production, recovery, and release for environmental concerns.

Ideal vapor compression refrigeration systems

Actual vapor-compression refrigeration systems

Performance of refrigeration systems

COP is a dimensionless quantity.

In the US, the performance is often given in dimensional terms:

Btu/(W-hr)--Energy Efficiency Ratio (EER)

Since 1.0 W-hr = 3.41 Btu

COP = EER/3.41

The anticipated performance over an average season is called the Seasonal Energy Efficiency Ratio (SEER), also in Btu/(W-hr).

Single-stage vapor compression refrigeration systems

Ideal vapor compression refrigeration systems

Heat pump systems

c

out

W

mQ

/

Heat pump systems

Refrigerants selection

Refrigerants selection

Refrigerants

The capital letter indicates the level of toxicity, and the Arabic numeral denotes the level of flammability.

Refrigerants

Refrigerants

 Chlorofluorocarbons (CFCs) (R-11, R-12, R-113, R-114, and R-115):

CFC production was banned in the US in 1995 due to the ozone depletion effects.

HCFCs (R-22, R-123, R-124, R-141b):

Represents a decreased threat to ozone layer due to the hydrogen molecule. Production was restricted in the US beginning January 1, 2004 and a worldwide ban is scheduled by 2030.

HFCs (R-125, R-134a, R-143a, and R-152a):

The least harmful to the ozone layer due to the lack of chlorine molecule. However, there is some pressure to reduce the use of these refrigerants because of their global warming potential.

Refrigerants

R-134a

Refrigerants

Refrigeration Equipment Components -Reciprocating compressors

Positive displacement compressor: increasing the pressure of a gas by reducing its volume.

Dynamic compressor: increasing the momentum of a gas followed by a conversion of the momentum into a pressure rise (centrifugal compressor).

adcba

a-b: Intake, the pressure below the intake pressure.b-c: Compression. Both valves are closed, pressure eventually reaches above the discharge pressure.c-d: Discharge, the pressure is maintained above the discharge pressure.d-a: Expansion, pressure eventually reduces to below the intake pressure, both valves are closed.

Refrigeration Equipment Components – Reciprocating compressor

Rotary Compressors

 The use of an eccentric rotor

Figures from Dossat and Horan

Rotary Compressors

The use of eccentric rotor to the housing plus vanes

Double helical rotary compressors

Double helical rotary compressors

Figures from Dossat and Horan

The interlobe spaces perform operations at different compression stages while trapped gas is passed from a larger interlobe space to a smaller one sequentially in an axial direction.

Double helical rotary compressors

Orbital compressors

Orbital compressors

Figures from Dossat and Horan

Orbital compressors

The trapped gas packet is sequentially compressed from a larger volume to a smaller volume in a radial direction.

Centrifugal compressors

Operating point of a compressor

 The compressor capacity is given in terms of evaporator load (Btu/hr: heat transfer rate in the evaporator, NOT flow rate), evaporator temperature , and condensing temperature in Fig. 15-7. When the compressor capacity and evaporator load (capacity) are plotted on the same chart, the intersection of the two curves is the operating condition of the compressor. The evaporator temperature is an indication of compressor load. A low evaporator temperature indicates the condenser may not be loaded.

From the compressor

From evaporator

Heat rejected in the condenser

Condenser

Evaporator

Expansion valves

 To reduce the pressure/temperature of the refrigerant, control the refrigerant flow rate, and maintain a sufficient vapor superheat at evaporator exit.

Expansion valves

Expansion Valve

Figures from Dossat and Horan

Compressor

inlet An exemplary setting point

When the evaporator load is increased (with a higher room air temperature, for example), more heat will be transferred to the refrigerant and the superheat at the evaporator outlet will increase. The control mechanism may allow a higher refrigerant flow rate, which will reduce the superheat.

T, room air or chilled water temperature.

eq

Expansion Valve

The expansion valve works to maintain a balance between the spring force, the evaporator outlet pressure and the bulb pressure (determined by the vapor superheat).

Compressor

inlet

Other expansion valves

Refrigerant piping

The lubricant should be compatible and miscible with the refrigerant.

The real single-stage cycle

3’- 3: Flow from the evaporator outlet to the compressor intake port (heat transfer from the environment to the refrigerant, slightly increasing enthalpy); 3-a: pressure drop through the intake valve; a-b: corresponds to the intake in Fig. 15-5; b-c: compression in Fig. 15-5; d -4: pressure drop through the discharge valve; and 4-4’: Flow from the discharge port to the condenser ( heat transfer from the refrigerant to the environment, slightly decreasing the enthalpy).

System Control

Chart 3 

Chart 4