1. Heat Transfer in a Room with a Hot Box
Scenario:
A room contains a box at 500°C, and the room is filled with still air.
Question:
Rank the modes of heat transfer (conduction, convection, radiation) at:
- Initial Stage
- Intermediate Stage
- Final Stage
Answer:
- Initial Stage:
- Radiation dominates due to high temperature difference and no air movement.
- Natural Convection begins to set in gradually.
- Conduction occurs only at points of direct contact (negligible for still air).
- Intermediate Stage:
- Natural Convection becomes more prominent as air starts circulating.
- Radiation continues but reduces slightly as temperature difference decreases.
- Conduction remains minimal.
- Final Stage (Thermal Equilibrium):
- Convection dominates as air movement becomes steady.
- Radiation and conduction become minor as temperature gradients reduce.
💧 2. Absorption & Absorption Tower Mechanics
a) Difference between Absorption and Adsorption:
| Parameter | Absorption | Adsorption |
|---|---|---|
| Mechanism | Bulk phenomenon (enters phase) | Surface phenomenon (sticks to surface) |
| Phases involved | Gas-liquid, liquid-liquid | Solid-gas, solid-liquid |
| Reversibility | Often difficult | Easier to reverse |
b) Counter-Current Absorption Tower:
- Diagram: Draw a vertical tower with:
- Gas Inlet (bottom)
- Liquid Inlet (top)
- Gas Outlet (top)
- Liquid Outlet (bottom)
- Flow directions: Opposite (counter-current)
c) Failure Conditions of Absorption Tower:
- Flooding due to high gas velocity
- Channeling due to poor packing
- Weeping or Entrainment due to incorrect flow rates
- Corrosion or material incompatibility
d) What if gas is replaced by liquid?
- The tower will no longer function as a gas-liquid absorber.
- System would need to be designed for liquid-liquid extraction, which has different flow behavior, solubility constraints, and residence time.
e) Is the tower suitable for liquid-liquid extraction?
- No, not directly.
- Requires modifications like:
- Mixer-settler design
- Specific internals for immiscible liquids
- Longer residence time
🌫️ 3. Humidification & Psychrometry
a) Define Humidification:
- The process of adding moisture (water vapor) to air.
b) How to Calculate Wet Bulb Temperature:
- Use a psychrometer (dry bulb + wet bulb thermometer).
- Use psychrometric chart or equations involving enthalpy balance.
c) What is Relative Volatility?
- It is the ratio of vapor pressures (or K-values) of two components.
- High relative volatility = easier separation via distillation.
d) Wet Bulb Depression:
- Difference between dry bulb and wet bulb temperature.
- Indicates moisture content in air.
e) Which Air is More Humid?
| Parameter | Air 1 | Air 2 | More Humid? |
|---|---|---|---|
| Wet Bulb Depression | Lower | Higher | Air 1 (more humid) |
| Relative Humidity | Higher | Lower | Air 1 (more humid) |
f) Graph – Wet Bulb Depression Across Seasons:
- Summer: High WBD (dry air)
- Monsoon: Low WBD (humid air)
- Winter: Moderate WBD
- Spring/Autumn: Transitional behavior
(Graph: X-axis = Months; Y-axis = Wet Bulb Depression)
🧯 4. Pumping System Analysis
Scenario:
- Pump is at 1st floor
- Draws water from ground floor (12 m below)
- Needs to deliver water 20 m above 1st floor
- Pump head = 40 m
Question:
Can it perform the task?
Solution:
- Total Static Head = 12 m (suction) + 20 m (delivery) = 32 m
- Since pump head (40 m) > total head (32 m),
✅ Yes, the pump can perform the task.
What is Priming?
- The process of filling the pump casing and suction line with liquid to remove air before starting.
Why is Priming Required?
- To avoid cavitation and ensure proper suction.
- Especially critical in centrifugal pumps, which cannot pump air.