# 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.

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?

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.

-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.

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.