In pharmaceutical industries many types of equipments are used for transfer of heat, they can be classified as follows,
Heat Exchangers.
Heat Interchangers.
Heat Exchangers:
These devices are used for transferring heat from a fluid (Hot Gas or Steam) to another fluid (Liquid) through a metal wall.
Heat Interchangers:
These devices are used for transferring heat from a One liquid to another liquid or one gas to another gas through a metal wall.
HEAT EXCHANGERS;
The equipment used for heat transferring are known as heat exchangers.
Some of the processes that involves heat transfer in pharmaceutical industries are:
Preparation of starch paste (in steam jacketed kettle).
Crystallization.
Evaporation.
Distillation.
Classification of heat exchangers
On the basis of transfer of heat, heat exchangers are classified as:
Direct transfer type:
The hot and cold fluids are separated by a metal wall through which the heat is transferred from hot fluid to cold fluid. E.g. shell and tube heater,
Storage type:
First a hot fluid is flown through a porous solid medium to heat the medium, then the cold fluid is flown through the hot solid porous medium to extract the heat from it.
This type of heat exchanger is not used in pharmaceutical industries.
Direct contact type:
Hot fluid is passed through the cold fluid and in this case the hot and cold fluids are not separated physically.
For example steam is bubbled through a cold liquid.
Tubular heater
Tubular heaters consist of circular tubes, one fluid flows through the inner tube, while the other flows through the outside space.
The heat transfer takes place across the wall of the tube.
Shell and Tube heater is the simplest form of tube heater.
1. Single Pass Tubular heater:
Construction:
It consists of a bundle of parallel tubes, which are relatively thin-walled. The ends of these tubes are fitted to two tube-sheets B1 and B2.
The bundles of parallel tubes are enclosed in a cylindrical shell or casing (C, made of cast iron) to which the tube-sheets (B1 and B2) are fitted.
Two distribution chambers D1 and D2 are provided at each end of the casing (C).
Cold fluid inlet (H) is fitted with distribution chamber D2 and hot fluid outlet (I) is fitted with the distribution chamber D1.
Steam inlet F, steam outlet K(called vent) and condensate outlet G are fitted to the shell.
Working:
Steam is introduced through the steam inlet F into the space surrounding the parallel tubes.
Heat is transferred to the cold liquid inside the tubes and steam is condensed.
The condensate is removed through condensate outlet G placed at the bottom of the casing. Non-condensable gases, if any, escape through the vent K provided at the top of the shell.
The fluid to be heated is pumped through the cold fluid inlet (H) into distribution chamber D1, flows through the tubes and collects in the distribution chamber D2.
Heat is transferred from the steam to the cold fluid through the metal wall. The hot fluid leaves the heater through outlet (I).
Advantages:
Large heating surface is packed into a small volume.
Disadvantages:
The velocities of the fluid flowing through these tubes are low because of the large cross-sectional area or surface area.
The expansion of the tubes and shell takes place due to differences in temperatures. This may lead to loosening of the tube sheets from the casing.
Initial cost and maintenance costs are very high.
2. Floating-head two-pass heater
Construction:
It consists of a bundle of parallel tubes.
They are enclosed in a shell (casing).
The right side of the distribution chamber is partitioned and fluid inlet and outlet are connected to the same chamber.
The partition is such that both have equal number of tubes.
On the left side the distribution chamber is not connected to the casing.
It is structurally independent, hence known as floating head.
The ends of the tubes are fitted with the floating head.
Casing is provided with a steam inlet, vent, and condensate outlet.
Working:
Steam is introduced through the steam inlet of the casing (shell). Steam heats the tubes. The condensate escapes through the condensate outlet fitted at the bottom of the casing. Non-condensable gas, if any, escapes through the vent of the casing provided at the top.
The cold fluid is introduced through the cold fluid inlet into the right-hand distribution chamber. The fluid flows through few tubes present in the lower part of the distribution chamber. The fluid reaches the floating head and changes direction and flows through the upper tubes again to the right-hand distribution chamber. The hot fluid is taken out through the outlet.
During this process the fluid in the tubes get heated due to heat transfer through the metallic wall.
Advantages:
Due to differences in temperature the tubes and shells may expand and the joints may get loose.
Since, the floating head part is independent of shell (or casing) hence the problem of loosening is prevented in this type of heater.
3. Liquid-to-liquid heat interchanger
Construction:
Construction is same as that of single-pass tubular heater, only difference is that it contains baffles to lengthen the path of flow of outer liquid.
A set of parallel tubes are fitted to two tube-sheets at two ends.
The tubes and the tube-sheets are placed inside the shell.
Cold liquid inlet is fitted to the left-hand side distribution chamber and outlet is fitted with the right-hand side of the distribution chamber.
Hot liquid is entering through the right-hand top side of the shell and leaving the shell through the outlet placed on the top, left-hand side of the shell.
Baffles consist of a circular metal sheet, with one side cut away. Baffles are placed inside the shell at appropriate places. Baffles have perforation on it through which the tubes pass.
Working:
The hot liquid is pumped from the left-top of the shell. The fluid flows through the shell (i.e. outside of the tubes) and moves down directly to the bottom (due to baffle), again moves up – like this it flows from the left to right-hand side of the shell.
Baffles increases the velocity of the hot fluid outside the tubes, which creates more turbulence. This reduces the film thickness at the outside of the tube and thus increases the film coefficient and thus heat transfer increases.
The baffles also get heated and add to the heat transfer to cold liquid.
The cold liquid is pumped through the inlet at the left-hand side distribution chamber. The liquid passes through the tubes and gets heated. The heated liquid is collected from the right-hand side distribution chamber.
Advantages: Heat transfer is rapid.
4. Double-pipe heat interchanger
N.B. In a liquid-to-liquid heat interchanger, the fluid to be heated is passed only once through the tubes before it is discharged, i.e. single pass.
The heat transfer in this case is not efficient. In a double-pipe heat interchanger number of passes can be increased as desired.
Construction:
In this case two pipes are used – one is inserted into the other. Cold fluid is passed through the inner tube. The outer pipe acts as a jacket for the circulation of hot fluid. All jacketed sections are interconnected.
Normally the number of pipe-sections are few. The length of the pipe is also less. The inner tubes may be made of glass and standard iron. The pipes are connected with standard return bends and the pipes are stacked vertically. The pipes may have longitudinal fins on its outer surface for better heat transfer.
Working:
The hot liquid is pumped into the jacketed section. It is circulated through the annular spaces between them and carried from one section to the next section. Finally it leaves the jacket. In this process the pipes get heated.
The liquid to be heated is pumped through the inlet of the inner tube. The liquid gets heated up and flows through the bend tube into the next tube-section and finally leaves the exchanger.
Limitation:
Double-pipe heat interchanger is economical when the heating surface area is less than 9 m2.
Commonly Asked Questions.
Write a short note on Shell and Tube heater.
Give classification of Heat exchangers.