U and Z Type Flat Plate Heat Exchanger

The heat exchanger is a simple mechanical device that is used to transfer heat between two fluids. Its only function is to facilitate the transfer of heat from the hot fluid to cold fluid and its design is made to maximize the transfer of heat. Base on the working mechanism of the heat exchanger there are two main types, one is a direct heat exchanger where the hot and cold fluid is in direct contact with each other, and the second is an indirect heat exchanger which hot and cold fluids are in separate channels. In the indirect types of heat exchangers, different types of design can transfer heat between fluids. These designs include

Tube in Tube Type Heat Exchanger

This type of heat exchanger transfers heat between the fluid using two tubes of different diameters. One tube with a smaller diameter with respect to the other tube is placed inside the larger tube and then fluids with different temperatures are allowed to flow in different tubes. Heat exchange between hot and cold fluid happens at the interface of the inner tube.

Shell and tube type Heat Exchanger

This type of heat exchanger transfers heat between fluids using more than one tube and a shell. Two or more tubes are placed parallel to each other inside a shell where hot fluid usually flows inside the shell and cold flow inside tubes. The temperature difference between the fluids across the tube thickness allows the transfer of heat from the hot fluid to the cold fluid. 

Plate Type Heat Exchanger

This type of heat exchanger transfer heat between fluid using a series of simple flat plates which are connected using gaskets. The spacing between the plates provides a channel for the flow of fluid where the two consecutive channels contain two different fluids. In this manner, each plate of heat exchanger has two different fluids on its different plates. 

Plate Heat Exchanger

The flat plate heat exchanger was designed and developed in 1920 using a number of plates in such a manner that each set of plates has some space in which it contains a liquid or gas matter for heat transfer. Two adjacent sets of places have two different fluid for heat transfer from which one fluid has a higher temperature than the other. Fluid usually enters and leaves perpendicular to the plate of heat exchanger using the holes provided at the end of plates. Movement of one fluid between different sets of plates is made possible by connecting every third set with the first one using pipes as shown in the figure below. In this manner, every set which contains hot fluid is surrounded by the cold fluid and every cold fluid set is surrounded by hot fluid (shah, 2003).

 

Based on the assembly of the plates there are two main types of flat plate heat exchangers. In the first type, the plate is attached to each other using simple nut and bolts where the gap between plate and joints is made fluid-tight by placing gasket. This type of heat exchanger can resist temperature up to 200 C and can hold the pressure of 25 bars.  In second type plates are simply brazed together where extended edges of plates provide gap required between plates for fluid flow. This type can work with a high temperature of up to 1100 C and can resist even high pressure (Bari, 2015). This type has a more compact design and is lightweight as compared to the first type. The material used for plates is stainless steel in most cases due to its high strength, corrosion resistance, and good thermal conductivity. In some cases, aluminum and copper can also be used. 

Working of the flat plate heat exchanger

Flat plate heat exchanger working follows the basic principle of thermodynamics and heat transfer that say that heat transfer from the region of higher potential to the region of lower potential by methods of conduction and convection. In a flat plate heat exchanger, the hot fluid present in a set of plates tries to transfer its heat to its side of the plate surface through convection. The heat from one side of a plate is transferred to its other side using conduction and from that opposite side of the plate, the heat is transferred to cold fluid using convection. The amount of heat transfer between hot and cold fluid depends on the overall heat transfer coefficient of the flat plate heat exchanger (shah, 2003). 

 

Types of Flat Plate Heat Exchanger

Base on the direction of flow of high and low-temperature fluid moving inside the channels of the flat plate heat exchanger is divided into 5 different classes which include U type, Z type, and Concurrent flow and Counter Concurrent flow.

U type flow  Heat Exchanger

U type flow in flat plate heat exchanger is one in which the flow of cold and hot fluid is in the form of the English alphabet U. This U shape flow starts from the inlet and ends at the outlet of the flat plate heat exchanger. This type of flow enables fluid to spend more time inside the channel of the flat plate heat exchanger which in return enables greater heat to absorb from hot to the cold fluid. This type of flow has one disadvantage and that is it has greater pressure drop due to difficulty in the fluid flow which reduces the overall performance of the system.

Z type of flow flat plate Heat Exchanger

Z type flow in flat plate heat exchanger is one in which the flow of cold and hot fluid is in the form of the English alphabet Z. This U shape flow starts from the inlet and ends at the outlet of the flat plate heat exchanger making the shape similar to Z. This type of flow enables fluid to move smoothly with a greater time of flow inside the channel of the flat plate heat exchanger which in return enables greater heat to absorb from hot to the cold fluid. This type of flow has one disadvantage and that is it has greater pressure drop due to difficulty in the fluid flow which reduces the overall performance of the system but is still better than U type flow.

Concurrent flow flat plate Heat Exchanger

The concurrent flow of fluid inside the flat plate heat exchanger is one in which the direction of motion of hot fluid and cold fluid moving inside the flat plate heat exchanger is concurrent with each other. The temperature profile in this type of flow shows that the temperature of hot fluid and cold fluid decreases and increases in a very similar manner. Both fluid exist heat exchangers at the temperature very close to each other.

Counter Concurrent flow flat plate Heat Exchanger

Counter Concurrent flow hot and cold fluid inside the channels of the flat plate heat exchanger is one in which the direction of motion of cold and hot fluid inside the flat plate heat exchanger is opposite to each other. The temperature profile in this type of flow shows that the temperature of hot fluid and cold fluid decreases and increases keeping the temperature difference constant. Both fluids exist heat exchangers at the temperature very different from each other.

Mode of heat transfer in the flat plate heat exchanger

Heat Transfer between fluid, solids and between fluid and solid happens because of any of the below mention two methods. That is conduction and convection mode of heat transfer. Both mode required medium to transfer heat from area of high temperature to area of low temperature.

Conduction in the flat plate Heat Exchanger

Transfer of heat from region of high temperature to the region of low temperature in a solid object body happen due to process of conduction. It depends on the solid body ability to transfer temperature and the total temperature difference between the high and low temperature areas (Bari, 2015). It can be calculated as

Q=k*A*∆T

Here

Q represents the heat transferred

k represents the material thermal conductance 

A represents the plate cross section area 

∆Trepresents the  temperature difference  

Convection in the flat plate Heat Exchanger

Transfer of heat from region of high temperature to the region of low temperature from fluid to solid body happen due to process of convection. It depends on the both materials ability to transfer heat and the total temperature difference between the high and low temperature areas. It can be calculated as

Q=h*A*∆T

Here

Q represents the heat transferred

h represents the material convection heat transfer coefficient 

A represents the plate cross section area in contact with fluid 

∆Trepresents the temperature difference between fluid and liquid 

Overall heat transfer coefficient of flat plate heat exchanger

As explained in the working of flat plate heat exchanger first there will be a convective heat transfer between hot fluid and plate then conduction heat transfer between opposite sides of plate and at last convective heat transfer between plate and cold fluid. The total amount of heat transfer by the system is defined by the overall heat transfer coefficient of the system. Overall heat transfer coefficient can be calculated as below (Bari, 2015).

1/U=1/h1 + x/k + 1/(h 2)

Where

U represents the overall heat transfer coefficient

h 1 represents the convective heat transfer coefficient between hot fluid and plate

K represents the conductive heat transfer coefficient of plate

h 2 represents the convective heat transfer coefficient between cold fluid and plate