1. What is heat exchanger? What are the types of heat exchangers?
A heat exchanger is defined as equipment which transfers the heat from a hot fluid to cold fluid.
Types
(i) Direct contact heat exchangers
(ii) Direct contact heat exchangers
(iii) Surface heat exchangers
(iv) Parallel flow heat exchangers
(v) Counter flow heat exchangers
(vi) Cross flow heat exchangers
(vii) Shell and tube heat exchangers
(viii) Compact heat exchangers
2. What do you meant by parallel and counter flow heat exchangers?
Parallel flow heat exchangers:
In this type of heat exchanger, hot and cold fluids move in the same direction.
Counter flow heat exchangers:
In this type of heat exchanger, hot and cold fluids move in parallel but opposite directions.
3. Write short notes on compact heat exchangers.
There are many special purpose heat exchangers called compact heat exchangers.
They are generally employed when convective heat transfer coefficient associated with one of the fluids is much smaller than that associated with the other fluid.
4. What is meant by LMTD?
We know that the temperature difference between the hot and cold fluids in the heat exchanger varies from point to point.
In addition various modes of heat transfer are involved.
Therefore based on the concept of appropriate mean temperature difference, also called logarithmic mean temperature difference, the total heat transfer rate in the heat exchanger is expressed as
Q=UA ΔTm
Where
U-Overall heat transfer coefficient in W/m2K
A- Area in m2
ΔTm – LMTD
5. What is meant by fouling factor?
We know, the surfaces of heat exchangers do not remain clean after it has been in use for some time.
The surfaces become fouled with scaling or deposits.
The effect of these deposits affecting the value of overall heat transfer coefficient.
This effect is taken care of by introducing an additional thermal resistance called the fouling resistance.
6. What is meant by heat exchanger effectiveness?
It is defined as the ratio of actual heat transfer to the maximum possible heat transfer.
Effectiveness = Q/Q max
7. What is meant by NTU? Give its expression.
Number of Transfer Unit (NTU) is a method which is used to determine the inlet or exit temperatures of heat exchangers.
NTU= UA / Cmin
8. Define emissive power.
It is defined as the total amount of radiation emitted by a body per unit time per unit area.
It is denoted as Eb and it is expressed in W/m2.
9. Define monochromatic emissive power.
The energy emitted by the surface at a given length per unit time per unit area in all directions is known as monochromatic emissive power.
10. What is meant by absorptivity, reflectivity and transmissivity?
Absorptivity:
It is the ratio between radiations absorbed to the incident radiation.
Reflectivity:
It is the ratio between radiations reflected to the incident radiation.
Transmissivity:
It is the ratio between radiations transmitted to the incident radiation.