How to Calculate Power Input into a Motor Compressor?
Motor compressors are the heart of refrigeration and air‑conditioning systems. They compress the refrigerant, raising its pressure and temperature, so that heat can be rejected in the condenser. To design, operate, or troubleshoot these systems effectively, engineers and learners must understand how to calculate the power input into the motor compressor.
🔍 Why Power Input Matters
- Energy Efficiency: Knowing compressor power helps evaluate system performance and energy consumption.
- System Design: Correct sizing ensures the motor can handle the load without overheating.
- Cost Control: Accurate calculations prevent overspending on electricity and maintenance.
📐 The Formula
The power input into the motor compressor is given by:
P(Comp) = M * (h2 - h1)
Where:
- P(Comp)= Power input into the motor compressor (kW)
- (M) = Mass flow rate of refrigerant (kg/s)
- (h1) = Enthalpy at compressor inlet (kJ/kg)
- (h2) = Enthalpy at compressor outlet (kJ/kg)
🧩 Step‑by‑Step Understanding
1. Mass Flow Rate (M)
- Represents how much refrigerant passes through the compressor per second.
- Measured in kg/s.
- Higher flow rate means more refrigerant is being compressed, requiring more power.
2. Enthalpy at Inlet (h1)
- Enthalpy is the energy content of the refrigerant.
- At the inlet, refrigerant is usually in a low‑pressure vapor state.
- (h1) is obtained from refrigerant property tables or software tools.
3. Enthalpy at Outlet (h2)
- After compression, refrigerant leaves at higher pressure and temperature.
- (h2) is also obtained from refrigerant property tables or charts.
4. Difference (h2 – h1)
- This represents the energy added per unit mass of refrigerant during compression.
- Multiplying by mass flow rate gives the total energy per second, i.e., the compressor power.
✅ Example Calculation
Suppose:
- Mass flow rate, (M = 0.05 , kg/s)
- Enthalpy at inlet, (h1 = 200 , kJ/kg)
- Enthalpy at outlet, (h2 = 240 , kJ/kg)
Step 1: Enthalpy Difference
h2 - h1 = 240 - 200 = 40 kJ/kg
Step 2: Power Input
[
P(Comp)= 0.05 * 40 = 2 kW
So, the motor compressor requires 2 kW of power input.
🎯 Key Takeaways
- Compressor power depends on mass flow rate and enthalpy difference.
- Enthalpy values are obtained from refrigerant property charts or software.
- Accurate calculation ensures efficient design, safe operation, and cost savings.