Refrigeration and Air Conditioning
Relative humidity is given by (where pv = Partial pressure of water vapour, pb = Barometric pressure, ps = Pressure of saturated air, pd = Pressure at dry bulb temperature, pw = Saturation pressure corresponding to wet bulb temperature, and μ = Degree of saturation)

μ/[1 - (1 - μ) (Ps/Pb)]
[Pv (Pb - Pd)]/ [Pd (Pb - Pv)]
0.622 Pv / (Pb - Pv)
None of these

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Refrigeration and Air Conditioning
In a vapour compression refrigeration system, a throttle valve is used in place of an expander because

It considerably reduces mass of the system
It leads to significant cost reduction
It improves the C.O.P., as the condenser is small
The positive work in isentropic expansion of liquid is very small

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