Mumbai University > Mechanical Engineering > Sem 8 > Refrigeration and air conditioning

Marks: 04M

Year: Dec 2016

i) Relative Humidity: It is defined as the ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour in saturated air at the same temperature and pressure. using perfect gas equation we can show that:

partial pressure of water vapour/saturation pressure of pure water vapour at same temperature

=$p_v/p_{sat}$

Relative humidity is normally expressed as a percentage. when RH is 100% then air is saturated.

ii)Ton of refrigeration: The practical unit of the refrigeration is expressed in terms of tonne of refrigeration A tonne of refrigeration is defined as the amount of refrigeration effect produced by the uniform melting of one tonne (1000 kg) of ice from and at $0^o$C in 24 hours.

since the latent heat of ice is 335 kJ/kg, therefore one tonne of refrigeration,

1TR = $1000 * 335$ kJ in 24 hours.

=$(1000*335)/(24*60)$ = 232.6 kJ/min

iii)Degree of saturation: The degree of saturation is the ratio of the humidity ratio W to the humidity ratio of a saturated mixture $W_s$ at the same temperature and pressure i.e.,

$μ=|W/W_s|_{t,p}$

iv)Dew point temperature: if unsaturated moist air is cooled at constant pressure, then the temperature at which the moisture in the air begins to condense is known as dew-point temperature(DPT) of air. An approximate eqaution for dew-point temperature is given by:

$DPT=(4030(DBT+235))/(4030-(DBT+235)lnΦ)-235$

where Φ is the relative humidity(in faction). DBT and DPT are in degree $C$. iv)Coefficient of performance: The coefficient of performance ( briefly written as COP) is the ratio of heat extracted in the refrigerant. it is also known as theoretical coefficient of performance. Thermostatically, $COP_{th}=Q/W$

where Q= Amount of heat extracted in the refrigerator.( or the amount of refrigeration produced or the capacity of a refrigeration produced, or the capacity of refrigerator)

W= Amount of work done.