Types Of Heat Exchanger | Basic Of Heat Transfer

INTRODUCTION TO HEAT EXCHANGER:
Heat exchangers are devices that facilitate the exchange of heat between two fluids that are at different temperatures while keeping them from mixing with each other. Heat transfer in heat exchangers involves convection in each fluid and conduction through the wall separating the two fluids. In order to account for the contribution of all the effects of convection and conduction, an overall heat transfer coefficient, U, is used in the analysis. Heat transfer rate depends on the temperature differences between the two fluids at the location and the velocity of the fluids (time of interaction) between the fluids.

TYPES OF HEAT EXCHANGERS

Due to the different types of applications for heat exchanges, different types of hardware and different configurations of heat exchanges are required. This has resulted to different designs of heart exchangers which includes and not limited to.

Double pipe heat exchanger (simplest heat exchanger)
Consists of two concentric pipes of different diameter. In application, one fluid passes through the pipe of smaller diameter while the other flows through the annular space between the two pipes. The flow of fluids can be arranged into:-

i). Parallel flow.
Both fluids (hot fluid and cold fluid) enter the heat exchanger at the same end and move in the same direction to leave at the other end as shown in the figure below.

(ii). Counter flow
In these types of arrangement, the cold and hot fluids enter the exchanger at opposite ends and flow in opposite directions as shown in the figure below:

The compact heat exchanger
This type of heat exchanger is designed to allow a large heat transfer surface area per unit volume. The ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density β. Heat exchangers with β >700 are classified as compact heat exchanger e.g. car radiator, human lung am0ongest others. They allow high heat transfer rates between fluids in a small volume. They are therefore best suited for applications with strict limitations on the weight and volume of heat exchanger. They are mostly used in gas-to-gas and gas-to-liquid heat exchanger to counteract the low heat transfer coefficient associated with fluid flow with increased surface area. The two fluids in this type of heat exchangers move in directions perpendicular to each other, a flow configuration referred to as cross-flow. This type of flow may be classified as unmixed or mixed.

i). Unmixed flow
Plate fins force the fluid to flow through a particular inter-fin spacing and prevent it from moving in the transverse direction.

ii). Mixed flow
The fluid is free to move in the transverse direction. The presence of mixing can have adverse and significant effects on the heat transfer characteristics of a heat exchanger.

Shell and tube heat exchanger
Contains a large number of tubes packed in a shell with their axes parallels to that of the shell. One fluid flows through the tubes while the other flows through the shell but outside the tubes. Baffles’ placed in the shell increases the flow time of the shell-side fluid by forcing it to flow across the shell thereby enhancing heat transfer in addition to maintaining uniform spacing between the tubes.These baffles are also used to increase the turbulence of the shell fluid. The tubes open to some large flow areas called header at both ends of the shell. These types of heat exchanger can accommodate a wide range of operating pressures and temperatures. They are easier to manufacture and are available at low costs. Both the tube and shell fluids are pumped into the heat exchanger and therefore heat transfer is by forced convection. Since the heat transfer coefficient is high with the liquid flow, there is no need to use fins. They can also be classified into parallel and counter flow types.

shell and tube heat exchanger with one shell pass and one tube pass

Plate heat exchangers
They are usually constructed of thin plates which may be smooth or corrugated. Since the plates cannot sustain as high pressure and or temperatures as circular tubs, they are generally used for small and low to moderate pressure/temperatures. Their compactness factor is also low compared to other types of heat exchangers. The plates can be arranged in such a way that there is crossflow i.e. the hot and cold fluids flowing in directions perpendicular to each other to enhance the heat transfer characteristic.