Question

Operation that causes increase in discharge pressure in Vapour Compression Refrigeration (VCR) system is

• A. Defrosting
• B. Air purging
• C. Decrease in condenser cooling water supply
• D. Flash vapour bypass

Hint:

Refrigeration System Dynamics: Inadequate cooling water flow in condenser $\longrightarrow$ Heat accumulation $\longrightarrow$ HIGH DISCHARGE (HEAD) PRESSURE.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Refrigeration condenser heat transfer and head pressure dynamics: reducing cooling water flow to the condenser lowers the heat transfer rate, forcing the refrigerant condensing saturation temperature ($T_{ ext{cond}}$) and corresponding discharge compressor pressure ($P_{ ext{discharge}}$) to rise sharply.
Key Formula or Approach:
\[ \dot{Q}_{\text{cond}} = \dot{m}_{\text{water}} C_p \Delta T \quad \xrightarrow{\dot{m}_{\text{water}} \downarrow} \quad T_{\text{cond}} \uparrow \implies \mathbf{P_{\text{discharge}} (\text{Head Pressure}) \uparrow} \]

Step 2: Detailed Explanation:
In refrigeration engineering and VCR system troubleshooting:
- Discharge (Head) Pressure: The high-side operating pressure created by the compressor discharging into the condenser, governed by the condensing temperature of the refrigerant:
1. Decrease in Condenser Cooling Water Supply (C):
- If the cooling water circulation rate drops (due to pump failure, scaled tubes, or clogged cooling tower nozzles), the condenser cannot reject the required heat load ($Q_c = Q_o + W$).
- The refrigerant vapor accumulates without condensing, driving up the condensing temperature ($T_{\text{cond}}$).
- Saturated vapor pressure rises exponentially with temperature, causing a severe Increase in Compressor Discharge Pressure (High Head Pressure Danger).
2. (Air purging removes non-condensable gases, which reduces discharge pressure; Defrosting clears evaporator frost).

Step 3: Final Answer:
Hence, Decrease in condenser cooling water supply causes increase in discharge pressure, matching option (C).