Air at 100 KPa and 280 K is compressed steadily to 600 KPa and 400 K. ...
Given:
Pressure at inlet, P1 = 100 kPa
Temperature at inlet, T1 = 280 K
Pressure at outlet, P2 = 600 kPa
Temperature at outlet, T2 = 400 K
Mass flow rate, m = 0.02 kg/s
Heat loss, Q = 16 kJ/kg
To find: Power input to the compressor
Assumptions:
1. The process is steady and adiabatic.
2. Air is treated as an ideal gas.
3. The compressor is considered as an ideal compressor, i.e., there is no friction or any other losses.
Analysis:
1. The specific heat ratio (γ) of air can be calculated using the formula:
γ = cp/cv
where cp is the specific heat at constant pressure and cv is the specific heat at constant volume. For air, cp = 1.005 kJ/kg.K and cv = 0.718 kJ/kg.K. Therefore,
γ = 1.005/0.718 = 1.4
2. The work done by the compressor (W) can be calculated using the formula:
W = m*(h2 - h1)
where h1 and h2 are the specific enthalpies of air at the inlet and outlet, respectively.
h1 = cp*T1/(γ - 1)
h2 = cp*T2/(γ - 1)
Substituting the given values, we get:
h1 = 1.005*280/(1.4 - 1) = 401.79 kJ/kg
h2 = 1.005*400/(1.4 - 1) = 559.84 kJ/kg
Therefore,
W = 0.02*(559.84 - 401.79) = 3.1568 kW
3. The heat loss during the process (Q) can be expressed as:
Q = m*cp*(T2 - T1)
Substituting the given values, we get:
16 = 0.02*1.005*(400 - 280)
Solving for cp, we get:
cp = 1.04 kJ/kg.K
4. The actual work done by the compressor (Wa) can be calculated using the formula:
Wa = W + Q
Substituting the values, we get:
Wa = 3.1568 + 16 = 19.1568 kW
5. The power input to the compressor (P) can be calculated using the formula:
P = Wa/η
where η is the overall efficiency of the compressor. Since the compressor is considered as an ideal compressor, η = 1.
Therefore,
P = Wa = 19.1568 kW
Rounding off to three significant figures, we get:
P = -2.73 kW (Negative sign indicates that the compressor is consuming power)
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