Safety

Whichever refrigerant is used it must be safe. This is dealt with in BS 4434: 1989, covering the design, construction and installation of refrigeration plant and systems. Refrigerants are classified in three groups:

1. These are non-inflammable in vapour form at any concentration in air at standard atmospheric pressure and 20°C. They have a low toxicity although when in contact with a flame or a hot surface toxic products of decomposition may form. Examples are: R11, R12, R13, R22, R113.

2. Toxicity is the dominant feature with these refrigerants and it is almost impossible to avoid a toxic concentration if an escape of refrigerant occurs. An example is: R717 (ammonia).

3. These are inflammable and are an explosive hazard, although with a low order of toxicity. Examples are: R170 (ethane), R290 (propane), R600 (butane).

Group 3 refrigerants should not be used for institutional or residential buildings, or those buildings used for public assembly.

Exercises

1. A refrigeration machine works on the simple saturation cycle. If the difference between the enthalpies of saturated liquid at the condensing and evaporating pressures is 158.7 kJ kg-1 and the latent heat of vaporisation under evaporating conditions is 1256 kJ kg-1, find the dryness fraction after expansion. Calculate the refrigerating effect.

Answers

0.1265 and 1097.1 kJkg"1.

2. Calculate the displacement of a compressor having 176 kW capacity if the refrigerating effect is 1097 kJ kg-1 and the volume of the suction gas is 0.2675 m3 kg-1. Assuming a volumetric efficiency of 75 per cent, what cylinder size will be needed if the speed is to be 25 rev s_1 and there are to be 6 cylinders with equal bore and stroke?

Answers

0.0429 m3 s"1 and 78.6 mm.

3. Find the change in entropy when a liquid refrigerant whose specific heat is 4.71 kJ kg“1 K“1 cools from 35.5°C to 2°C. If 12.64 per cent liquid then vaporises and the latent heat of that process is 1256 kJ kg-1, what further change in entropy occurs? State the total change in entropy per kg of refrigerant for the cooling and partial vaporisation.

Answers

-0.541 kJ kg“1 K-1, +0.577 kJ kg’1 K"1 and +0.036 kJ kg’1 K“1.

4. A 4-cylinder 75 mm bore x 75 mm stroke compressor runs at 25 rev s_1 and has a volumetric efficiency of 75 per cent. If the volume of the suction gas is 0.248 m3 kg-1 and the machine has an operating efficiency of 75 per cent, what power will be required on a simple saturation cycle when the difference between the enthalpies of the suction and discharge gases is 150 kJ kg-1? If the refrigerating effect is 1087 kJ kg-1 what is the output of the machine in kW of refrigeration? State the coefficient of performance.

Answers

20 kW, 108.7 kW of refrigeration and COP = 7.25.

5. Water is used in a simple saturation vapour compression refrigeration cycle and the evaporating and condensing temperatures and absolute pressures are 4.5°C with 0.8424 kPa and 38°C with 6.624 kPa, respectively. Assume that water vapour behaves as an ideal gas with cp/cv = 1.322 and calculate the discharge temperature if compression is isentropic. Find also the kW/kW if the refrigerating effect is 2355 kJ kg-1.

Answers

(i) 186°C, (ii) 0.1454 kW per kW of refrigeration.

Notation

Symbol

Description

Unit

COP

Coefficient of performance

C

Constant in equation pVn = c (see section 9.8)

C

Specific heat of liquid

KJ kg“1 K"1

Cp

Specific heat of gas at constant pressure

KJ kg“1 KT1

CV

Specific heat of gas at constant volume

KJ kg“1 K-1

D

Diameter

Mm or m

F

Dryness fraction

8

Local acceleration due to gravity

M s-2

H

Enthalpy

KJ kg“1

Hic

Enthalpy of saturated liquid at condensing pressure

KJ kg“1

Enthalpy of saturated vapour at condensing pressure

KJ kg“1

Hd

Enthalpy of saturated vapour at discharge condition

KJ kg-1

H

Nve

Enthalpy of saturated vapour at evaporating pressure

KJ kg 1

M

Mass of refrigerant

Kg

M

Mass flow rate of refrigerant

Kg s 1

N

Exponent in equation pV1 = c (see section 9.8)

P

Pressure

Pa or kPa

Pc

Condensing pressure

Pa or kPa

Pa

Discharge pressure

Pa or kPa

Pt

Evaporating pressure

Pa or kPa

Q

Rate of heat transfer to the system

Wor kW

Qc

Rate of heat rejection at condenser

Wor kW

Gra

Actual refrigeration capacity

WorkW

Q

Specific heat energy

KJ kg-1

Heat rejected at condenser

J kg-1 or kJ kg

%

Heat transferred to cylinder jacket during compression

J kg-1 or kJ kg"1

<7r

Refrigerating effect

J kg 1 or kJ kg-1

R

Particular gas constant

KJ kg“1 K1

S

Entropy

KJ kg”1 K“1

S

Entropy of saturated vapour at evaporating pressure

KJ kg“1 K-1

Si

Entropy of saturated vapour at condensing pressure

KJ kg“1 K"1

*3

Entropy of saturated liquid at condensing pressure

KJ kg’1 KT1

T

Absolute temperature

K

Tc

Absolute condensing temperature

K

Td

Absolute discharge temperature

K

Te

Absolute evaporating temperature

K

Tc

Condensing temperature

°c

Evaporating temperature

°c

V

Volume

M3

Vd

Volume at discharge condition

M3

Ve

Volume of saturated vapour at evaporating pressure

M3

V

Velocity

M s"1

Vs

Specific volume

M3 kg“1

V

Volumetric flow rate

3 “1 m s

W

Rate of work done by the system

W or kW

Wr

Power needed for compression

WorkW

Wr

Work done in compression

J kg-1 or kJ kg

Z

Elevation above a reference level

M

Y

Cp/Cv

Volumetric efficiency

%

Posted in Air Conditioning Engineering


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