Question

The calandria and vapour separators are at an elevated position than the milk pumps, for the purpose of

• A. Convenient height for inspection
• B. Providing positive suction pressure
• C. Reducing number of pumps
• D. Reducing the steam consumption

Hint:

Vacuum Pump Hydraulics: Elevating evaporator vessels provides POSITIVE SUCTION PRESSURE (NPSH) to prevent pump cavitation when pumping boiling liquids under vacuum.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Pump net positive suction head (NPSH) under vacuum: liquid milk inside the evaporator boils at its saturated vapor pressure under deep vacuum ($P_{ ext{abs}} = 10-30 ext{ kPa}$); elevating calandrias 3 to 6 meters above extraction pumps creates a gravity static head that provides adequate NPSH available ($NPSH_a > NPSH_r$) to prevent catastrophic pump cavitation.
Key Formula or Approach:
\[ NPSH_a = \frac{P_{\text{surface}} - P_v}{\rho g} + \mathbf{h_{\text{static}} (\text{Elevated Height})} - h_f = \mathbf{+ h_{\text{static}} \implies Prevents \text{ } Cavitation} \]

Step 2: Detailed Explanation:
In dairy plant layout engineering and vacuum extraction pump hydraulics:
- In falling film evaporators, milk circulates and concentrates under deep vacuum ($10 - 20\text{ kPa}$ absolute pressure).
- Inside the vapor separator sump, the liquid milk is boiling at its exact saturation vapor pressure ($P_{\text{surface}} = P_v$).
- Consequently, the pressure term $(P_{\text{surface}} - P_v)$ equals zero:
1. If an extraction pump were located at the same horizontal level, the slight pressure drop at the impeller suction eye would instantly vaporize milk into steam bubbles, causing violent cavitation, vapor locking, and impeller destruction.
2. By placing the calandria and vapor separators at an Elevated Position (3 to 6 meters above floor level), the vertical liquid column creates a significant positive static gravity head ($+h_s$).
3. This Provides positive suction pressure (Net Positive Suction Head NPSH) (B) ensuring bubble-free liquid feed into the pump suction.

Step 3: Final Answer:
Therefore, calandrias are elevated for Providing positive suction pressure, corresponding to option (B).