Question

Describe the following:
(1) Efects of natural convection at heat exchanger
(2) Frank's laws and color temperature
(3) Geometric factor and overall interchange factor
(4) Factors Affecting Evaporator Capacity

Answer 1

1) In natural convection, the fluid motion occurs by natural means such as buoyancy. Since the fluid velocity associated with natural convection is relatively low, the heat transfer coefficient encountered in natural convection is also low.
Natural convection on a surface depends on the geometry of the surface as well as its orientation. It also depends on the variation of temperature on the surface and the thermophysical properties of the fluid.

2)

frank's law:  It states that temperature differences cause heat flow. Frank's view states that a body's acceleration is caused by the mass of and the net force on the body. Likewise, Fourier's law of heat conduction says that the heat flux density at a given location is proportinal to the negation of temperature gradient there.
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source.

3)

Heat transfer rate in the exchanger is represented by

q = U * A * F * LMTD

here F (< 1) is interpreted as a geometric correction factor, that when applied to the LMTD (Log Mean Temperature Difference) of a counter flow heat exchanger, provides the effective temperature difference of the heat exchanger under consideration.

It is a measure of the heat exchanger’s departure from the ideal behavior of a counterflow heat exchanger having the same terminal temperatures.

overall interchange factor: The overall interchange factor measures the overall types of shapes used for conductive and convective transfer of heat. It is commonly applied to the calculation of heat transfer in heat exchangers, but can be applied equally well to other problems.

4) Capacity is given as Q = UA(T2 - T1)
factors affecting are:
MATERIAL
TEMPERATURE DIFFERENCE
VELOCITY OF REFRIGERANT
THICKNESS OF THE EVAPORATOR COIL WALL
CONTACT OF SURFACE AREA