Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 153
Absolute pressure (bar)
9.6
3.6
Liquid
region
3
4
Critical point 120°C, 45 bar
Expansion
Liquid–vapour
mixture region
Condensation
Evaporation
Specific enthalpy (kJ/kg)
2
Compression
1
Superheated
vapour region
5.17 Pressure–enthalpy diagram for a refrigerant showing the vapour-compression cycle.
The pressure rise produced through the compressor is dissipated in friction through the fine
orifice in the valve. Such a sudden pressure drop is almost an adiabatic thermodynamic process.
This is represented by the vertical line 3–4 on the pressure–enthalpy diagram. Some heat loss
from the valve body takes place so that 3–4 will be slightly curved. Condition 4 is at the lower
pressure of the evaporation process, where the refrigerant temperature has dropped to 5 ◦ C.
Some of the refrigerant liquid has flashed into vapour.
The liquid and flash vapour mixture flows through the evaporator, where it is completely
boiled into vapour and is then given a small degree of superheat (path 4–1). It then enters the
compressor as dry low-pressure superheated vapour at 20 ◦ C. A suction temperature-sensing phial
controls the refrigerant flow rate by means of a liquid-filled bellows on the thermostatic expansion
valve. This matches refrigerant flow to the refrigerating effect required by the air-conditioning
system and ensures that liquid droplets are not carried into the compressor, where they could
cause damage.
Large plant has refrigerant pressure controllers, which reduce compressor performance by
unloading some cylinders. Lubricating oil contaminates the refrigerant leaving the compressor.
This is separated gravitationally and returned to the crankcase.
The pressure–enthalpy diagram depicts the reverse Carnot cycle. In the other direction, the
cycle is for an internal combustion engine. The theoretical ideal coefficient of performance is
given by,
Ideal COP = T 1
T 2 − T 1
where T 1 is the evaporation absolute temperature (K) and T 2 is the condensation absolute
temperature (K). With the temperatures previously used,
Ideal COP =
273 + 5
(273 + 40) − (273 + 5)
= 7.94
Friction losses from fluid turbulence, heat transfers to the surroundings, and mechanical and
electrical losses in the compressor all reduce this value to 2–4 in commercial equipment. The
electricity consumption of fans and pumps adds to the running costs.