HEAT EXCHANGER

1. Which of the arrangement of heat exchangers is better, (i) parallel flow, (ii) Counter

flow. Explain the reasons.

Counter flow HEX is better because of the following reasons:

Average temperature difference is more. There is less variation in temperature difference all the length of the HEX. Higher rate of heat transfer Economical

2. What is meant by Duty of a heat exchanger?

The rate of heat to be transferred is the duty of a heat exchanger i.e. 50kW duty heat exchanger.

3.  What are plate heat exchangers?

It consists of parallel plates. Fluid flowing in alternate parallel plates, fluid enter from same side of plates say lower side and leaves at the upper side. It is like having distributors on both sides. After flowing through say four plates the fluid enters another set of parallel plates. In these it enters at the top and leaves at the bottom. These are suitable for any single or multi-phase heat transfer.

4. Explain when one fluid is undergoing phase change, the direction of flow is immaterial for finding LMTD.

When one fluid is undergoing phase change, its temperature remains constant. The two end temperature differences in parallel and counter flow will be same. Hence during phase change of a fluid, direction of flow makes no difference in the rate of heat transfer.

5. What is dry bulb temperature, wet bulb temperature?

Dry bulb is the temperature of ambient air measured by thermometer. It is called “Dry bulb temperature” because the temperature shown by thermometer is not affected by the moisture content of the air.

The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air.

6. What is dew point and why it is important in instrument air?

The Dew Point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature the moisture will stay in the air. This is ensured by keeping the dew point below -40OC or below.

7. What is the use of baffles in heat exchangers?

Baffles installed in a heat exchanger acts as an obstruction in the flow path of shell side liquid. It reduces the effective cross section area through which the liquid travels. Due to this the velocity and turbulence of liquid is increased thus resulting in high heat transfer.

8. Why baffles are used in reactors?

The purpose of baffles in reactors is as same as that of heat exchangers. Here also they increase

turbulence thus causing higher rate of heat transfer, mixing thus ultimately reaction rate.

9. Why SS are not corrosive?

The SS (stainless steel) contains some percentage of chromium. The chromium reacts with oxygen

(which is present in air) to form Cr2O which prevents further oxygen to react with Iron (Fe) of SS to

form rust.