Refrigeration systems

COLLEGE OF ENGINEERING, DESIGN, ART AND TECHNOLOGY(CEDAT)

SCHOOL OF ENGINEERINGDEPARTMENT OF MECHANICAL ENGINEERING

MEC 4205: AIR CONDITIONING AND REFRIGERATION

UNIVERSITYMAKERERE

COURSE FACILITATORMr. Aggrey MwesigyeAssistant [email protected] 223 (Old building)

AIR CONDITIONING AND REFRIGERATION (MEC 4205)

Lecture notes compiled from the following textbooks

1.Andrew, D.A., Carl, H.T., Alfred, F.B. (2000). Modern Refrigeration and Air Conditioning. The Goodheart-Willcox Company Inc.2.Jones, W.P. (2005). Air Conditioning Engineering (5th Ed.). Butterworth Heinemann 3.Dossat, R.J. (1991). Principles of Refrigeration (3rd Ed.). Prentice Hall, Englewood Cliffs, New Jersey4.Wilbert, F. S., Jerold W.J. (1982). Refrigeration and Air Conditioning (2nd Ed.). McGraw-Hill International Editions5.Whitman, Johnson, Tomczyk. Refrigeration and Air Conditioning Technology (5th Ed.).6.Yunus, A.C., Michael A. B. (1998). Thermodynamics: An Engineering Approach, (3rd Ed.). McGraw-Hill Inc.

MULTI-STAGE REFRIGERATION SYSTEMS

AIR CONDITIONING AND REFRIGERATION (MEC 4205)

Basic Vapour Compression Refrigeration Systems- A liquid boils and condenses: – the change between the liquid andgaseous states – at a temperature which depends on its pressure,within the limits of its freezing point and critical temperature.-In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again. - In a vapor compression cycle as the name implies, liquid refrigerants are used which are alternately evaporated and condensed.- The cycle makes use of boiling and condensing of a working fluid at different temperatures and pressures- Heat is put into the fluid at a lower temperature and pressure and provides latent heat to make it boil and change to a vapor. Note: Boiling temperature of a liquid increases with pressure

REFRIGERATION SYSTEMS

Refrigeration SystemsVapour Compression Refrigeration System-The vapour is then mechanically compressed to a higher pressure and a corresponding saturation temperature at which its latent heat can be rejected so that it changes back to a liquid-The total cooling effect will be the heat transferred to the working fluid in the boiling or evaporating vessel.

Figure: Basic VPC Refrigeration System

Refrigeration SystemsVapour Compression Cycle (VPC)- Other components of the vapour compression cycle are added for efficiency, cost reduction, safety and convenience. They Include: (a)Auxiliary Components: - Filter/drier

- Receiver ( Stores excess refrigerant) - Oil separation ( Separates oil from refrigerant)

- Accumulator (Removes excess liquid from the vapour) - Moisture indicator

- Guages(b) Controls

- Thermostat(c) Electricals

- Motors.

Refrigeration SystemsVapour compression refrigeration system

Additional Components Gauge

Oil SeparatorFilter/Dry

Moisture indicator Motor

Expander Compressor

Gauge Gauge

Accumulator

Condenser

Evaporator

Receiver

Figure: Additional Components of the Vapor Compression Cycle

Refrigeration SystemsVapour compression refrigeration system

Figure: Refrigeration systems

Refrigeration SystemAnalysis of the single –stage vapour compression system(a)The Ideal Cycle (Carnot Refrigeration Cycle or Reversed Carnot Cycle)

3 2 PH PH

Expander Compressor T PL TH 3 2

PL

4 1 TL 4 1

SFigure : Basic components T-S Diagram

Condenser

Evaporator

PH Condensing PressurePL Evaporating Pressure

Refrigeration SystemsVapour Compression Cycle Analysis (Reversed Carnot Cycle)Stage 1-2-Vapour is compressed isentropically from a low pressure temperature to a higher pressureStage 2-3-Vapour is passed through a condenser in which it condenses at a constant pressure. During which the refrigerant rejects heat to another mediumStage 3-4In the expander, the refrigerant expands theoretically isentropically back to the original low pressure, low temperature Stage 4-1At the lower temperature side, a low pressure is maintained allowing the refrigerant to be vaporized

Refrigeration SystemsPerformance Criteria- Refrigeration effectQ4-1 = h1 – h4

This is the heat to be extracted in kJ/kg (should as large as possible)Refrigeration Capacity is the product of the refrigeration flow rate and the refrigeration effect in kW-Compression specific inputW1-2 = h2 – h1

This is the work input per unit flow (Should as low as possible)Power input into the compressor is the product of the refrigerant mass flow rate and the work input.-Heat rejected by the refrigerantQ2-3 = h2 – h1

This is the heat generated from the condenser

Refrigeration SystemsPerformance Criteria- Net expenditure of workWnet = W1-2 – W3-4

Since the expander does not do or take in any work usually Wnet = W1-2

- Coefficient of PerformanceThis is the measure of how efficient the refrigeration system isIt is given by C.O.PR =

For a heat pump the coefficient of performance is given as

C.O.PH = ; C.O.PH = C.O.PR + 1

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Refrigeration SystemsPerformance Criteria- Units of RefrigerationUsed to specify the capacity of a refrigerator and is given as TR (Tons of refrigeration) defined as the amount of refrigeration effect obtained from 1 ton of ice at 0 0C melting in 24 hours

1T.R = 3.5kWExampleA Carnot refrigeration cycle using Freon 12 operates between upper and lower temperatures of 400 C and -100 C, respectively. If the cycle handles a load 10kW determine;-The coefficient of performance- The mass flow rate of the refrigerant- The power required to drive the compressor- The work done by the expander

Refrigeration SystemsLimitations of the Carnot Refrigeration CycleFrom the T-S diagrams compression is done in vapour liquid mixture which has got several limitations.-Over working the compressor-Difficulty in compressing fluids-The lubricating fluid might mix with the refrigerant which will be discharged to the evaporator thus reducing the cooling effect at the evaporator.

Refrigeration SystemsModifications of Carnot Refrigeration CycleSince the refrigerant entering the compressor is pure saturated vapour or superheated and the one going back to the evaporator from the condenser should be totally liquid or sub-cooled liquid this gives many modifications of the Carnot refrigeration cycle and the expander replaced by a throttle valve.

PH 2

T P TH 3 PH 3 2

TL 4 1 PL 4 1

S h Figure: Typical T-S and P-h diagrams for standard vapour compression cycle

Refrigeration SystemsModifications of Carnot Refrigeration CycleThe modifications ensure1.Dry compression or super heating2.Use of throttle valve to ensure constant enthalpy of the refrigerant entering the evaporator3.And or Sub-cooling so that as much heat as possible is picked from the refrigerated spaceAnalysis of the standard vapour compression cycle-Refrigeration effect Q4-1 = h1 - h4

-Compression specific inputW1-2 = h2 - h1

-Heat rejected by the refrigerantQ2-3 = h2 – h3

Refrigeration SystemsModifications of Carnot Refrigeration Cycle-Coefficient of performance C.O.PR =

-Mass flow rate

-Volume of refrigerant drawn into the compressor is given by

-v1 is the suction volume = specific volume of the refrigerant at state 1.

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Refrigeration SystemsExample- A standard vapour compression cycle developing 50kW of refrigeration using refrigerant 22 operates with a condensing temperature of 350C and an evaporating temperature of -100 C. Calculatei.The refrigerating effect in kJ/kgii.Mass flow rate of the refrigerantiii.Power required by the compressor in kWiv.Coefficient of performancev.Volume flow rate measured at the compressor suction

Refrigeration SystemsActual vapour compression cycle-The actual vapor compression cycle suffers from inefficiencies compared with the standard cycle. The essential differences between the actual and the standard cycle appear in the pressure drops in the condenser and evaporator, in the sub-cooling of the liquid leaving the condenser, and in the superheating of the vapor leaving the evaporator.

- The sub-cooling of the liquid in the condenser is a normal occurrence and serves the desired function of ensuring that 100% liquid will enter the expansion device. - Superheating of the vapour usually occurs in the evaporator and is recommended as a precaution against droplets of liquid being carried over into the compressor. - The final difference is that the compression is no longer isentropic because of the inefficiencies due to friction and other losses

Refrigeration SystemsVapour compression cycle with sub-cooling-The performance of the cycle can be improved by liquid sub-cooling after condensation, through heat rejection at the high pressure side before expansion; this decreases the quality of the mixture at inlet to the evaporator thus increasing the refrigerating effect by the amount of heat rejected in the sub-cooling process. Sub-cooler T 2 P 4 3 2 2s

3 4 3 2s 2

4 5 1

5 1 5 1

S

Condenser

Evaporator

Figure: Sub-cooled vapour compression system with irreversibilities

Refrigeration Systems- Isentropic efficiency

- The common causes of irreversibilities that occur in the compressor are: - Fluid friction which causes pressure drops - Heat transfer to or from the surroundings.-Isentropic work done on the compressor remains unchanged with sub-cooling- Refrigeration effect increases to h1 – h5 , the increase in the refrigeration effect being h3 – h5

- The heat rejected by the condenser also increases to h2 – h4 by h4 – h3

- h4 h5 and is equal to hf at the sub-cooling temperature or from

h4 = (hf at TH ) – C(TH – T3) C is the specific heat capacity of the refrigerant

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Refrigeration Systems- Effect of superheating suction vapour

In the basic cycle the vapor is assumed to leave the evaporator and reach the compressor as a saturated vapor. In an actual cycle however, the vapor usually becomes superheated before compression. Super heating also increases the refrigeration effect of the system.

T P

4 1 4 1

S hFigure: Actual vapor compression cycle with suction vapor super heating

The enthalpy of superheated vapor is determined by interpolation

Refrigeration SystemsExample- A plant using R717 Ammonia operates between an upper and lower temperatures of 300 C and -150 C respectively. The refrigerant enters the compressor as a saturated vapour and is sub-cooled to 180 C before entering the throttle valve. Findi.The refrigeration effect ii.Coefficient of performanceiii.What would the result be if Freon 12 were the refrigerant instead of ammonia? What is your comment.

Refrigeration SystemsEffect of operating conditions on the practical single vapor compression systems- Evaporator pressure (PL) A decrease in the evaporator pressure leads to:

- Reduction in the refrigeration effect- Increase of compressor work- x4 (throttling loss) increases. But the interest would be to have a small x4 as possible. (x4 is the useless vapor created due to throttling measured by the quality at 4.- TH (Discharge temperature) increases. This increased temperature leads to power loss, safety of compression is compromised and the compression life is shortened due to wear.

- Condensing pressure (PH )An increase in condensing pressure leads to: -Reduction of the refrigeration effect, an increase of compressor work

input, reduced coefficient of performance, an increase in x4 and an increase in the discharge temperature.

Refrigeration SystemsEffect of operating conditions on the practical single vapor compression systems- Vapour superheatingLeads to:

- an increase in the refrigeration effect, and an increase in the discharge temperature. Work input to the compressor may or may not change. - Liquid sub-coolingLeads to

- An increase in the refrigeration effect, a reduction in x4, TH remains constant so is the compressor work input.

- In order to improve on the performance of the single stage and or multistage compression systems , there must be a balance between the savings made and the capital costs to make the investments