Heat Exchangers and Their Types - Tube, Shell, Regenerative and Plate Heat Exchangers

There is not a lot of public knowledge about heat exchangers; they are generally only discussed in mechanical and chemical engineers circles. Yet they are found all over the world in common appliances like air conditioners and refrigerators, the radiator in motor vehicles is an another example.

The basic principle behind heat exchangers is the transferring of heat from one medium to another. For example the car radiator allows hot fluid from the engine to be cooled when the fins of the radiator radiate the heat into the air that flows over its surface. Some factors that influence the efficiency of heat exchangers are the fluids rate of flow and the total surface area of the wall that separates the fluids. To increase the surface area of the contact wall fins and corrugations are added, this is an effective measure.

Of The 4 types of heat exchanger the shell and tube configuration is most common and also relatively easy to maintain. This system has multiple tubes with fluid running inside them and simultaneously running over them. This allows a heat transfer to take place.

Another one is regenerative type that is common in gas cooling applications has the effect of taking heat produced in one process and transferring it to another also known as heat recovery. Plate and intermediate fluid exchangers are the other types.

 The plate configuration uses the same technique as the shell and tube just here the fluid is made to flow through layers of cavities between multiple plates the result is a large surface area. This configuration is considered to have the best thermodynamic performance for this application and it is widespread in the engineering world. But it is more time consuming and labour intensive to fabricate than a shell tube heat exchanger.

The way the fluid flows through the these equipments has been divided into 3 major groups. These flow arrangements are cross-flow, counter-flow and parallel-flow. Of these the counter-flow configuration transfers the greatest amount of heat.

Practical Information on Shell and Tube Heat Exchangers

Shell and tube heat exchangers can be used in a variety of industries for a number of purposes.  The heat exchanger's composition consists of a number of tubes.  Some of the tubes contain fluid that is heated or cooled depending on a particular job.  A second set of tubes manipulate the first so the exchanger can either give or absorb heat.  Shell and tube heat exchangers are usually implemented in high-pressure endeavors. 

Heat exchangers are usually composed of fluoropolymers.  Fluoropolymers such as PTFE, PFA, FEP, and PVDF are used in a variety of jobs due to their versatile and tenacious nature.

Those in the market for shell and tube heat exchangers need to take a few elements into account before making a purchase.  Consider the following:

-    The diameter of the tubing can be manipulated by the provider.  A main point to consider is the nature of the particular fluids used in the tubes.  Smaller-sized tubes will warrant faster cleaning, yet bigger tubes may be less economical and less compact regarding space.

-    The thickness of tubes relates to several factors.  Corrosion, flow resistance, axial strength, pressure, and availability of spare parts relates to a heat exchanger's tube thickness.

-    Heat exchanger cost is influenced by shell diameter and tube length.  Clients who are concerned about cost ask for exchangers which provide the longest tube length without compromising its efficiency.  The possibility for long tubes may be limited due to space, specific job specifications, and replacement possibilities.

-    Corrugation of tubes influences performance.  Corrugated, inner tubes allow for increased turbulence of fluids, in turn delivering better performance.

-    'Tube layout' refers to how a heat exchanger's tubes are positioned within the shell.  To date, there are four, main layouts to consider:  triangular, rotated triangular, square, and rotated square.  Triangular tubing facilitates better heat transfer while square tubing allows for a longer period of cleanliness.

-    'Tube pitch' refers to the distance between centers of individual but connected tubes.  A general rule states a tube's pitch should not be less than 1.25 times the tube's outer diameter.

-    'Baffles' are used in shell and tube heat exchangers to direct fluid flow across the tube bundle.  Baffles prevent the tubes from sagging, and can also prevent them from vibrating.  Baffle spacing is important in regards to pressure drop and heat transfer.  Baffles spaced closely causes a greater pressure drop, yet placed too far apart may cause cooler spots between them.

Shell and Tube Heat Exchanger Experiment

Introduction

The process of heat exchange between two fluids which are at different temperatures and flow rates occurs in many engineering applications. Several types of heat exchangers have been widely used in the industry. Shell-and-Tube heat exchangers are among the common types of heat exchangers.  A basic schematic for a single pass shell-and-tube heat exchanger is shown in Figure 1.  The stream to be cooled enters the tube side and is distributed amongst the tubes shown with red arrows.  

The stream that cools the liquid is shown in blue enters on the shell-side and flows perpendicular to the tube bundle for maximum heat transfer.  The shell-side flow passes around baffles placed around the tube bundle in order to increase both the residence time of the fluid around the tube bundle as well as to promote turbulence in order to maximize the efficiency of the heat exchanger. 

Heat exchangers are typically classified according to flow arrangement. In the parallel-flow heat exchanger, the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end. In the counter-flow arrangement, the hot and cold fluids enter the heat exchanger at different ends and flow in opposite directions. Each fluid arrangement leads to different heat rates and the calculations are different accordingly [1].

    

 

Figure 2 illustrates the Heat Exchanger experiment apparatus.  Based on long-term setup of the apparatus, the hot flow passes through the tube and the cold flow passes through the shell. Different configuration of valves can result in parallel and counter flows.  The hot and cold flow rates are measured by two flow meters, located along the incoming flows.

In order to measure the temperature of fluids and heat transfer surfaces, ten thermocouples have been installed in this apparatus. The measured temperature of each thermocouple is shown on the readout when the selector switch points to the thermocouple number. The thermocouples are located in different positions to measure the temperatures according to Table 1.

Table1. Thermocouples

Thermocouple	Measured Temperature	Thermocouple	Measured Temprature
1	Tube Inlet	5	Shell Inlet
2	Tube-side heat transfer surface at 16” *	6	Shell-side heat transfer surface at 16” *
3	Tube-side heat transfer surface at 19” *	7	Shell-side heat transfer surface at 19” *
4	Tube-side heat transfer surface at 21” *	8	Shell-side heat transfer surface at 21” *
10	Tube Outlet	9	Shell Outlet

* measured from the tube side.

Laboratory Procedure:

Parallel Flow Experimental Setup

1. Open the valves 1, 3, 4, 6, and 7 all the way.
2. Close valves 2 and 5.
3. Turn on the main power.
4. Turn on hot and cold water supply all the way.
5. Close valve 1 until it reads 2 GPM.
6. Close valve 3 until it reads 20% (2 GPM).
7. Wait at least 15 minutes so that the system reaches steady state.
8. Record temperatures of the system at steady state according to Table 2.
9. Turn off the hot and cold water supplies.
10. Proceed to the next configuration.

Counter Flow Experimental Setup

1. Open valves 1, 2, 3, 5, and 7 all the way.
2. Close valves 4 and 6.
3. Turn on the hot and cold water supplies all the way.
4. Close valve 1 until it reads 3 GPM.
5. Close valve 3 until it reads 30% (3 GPM).
6. Wait at least 15 minutes so that the system reaches steady state.
7. Record temperatures of the system at steady state according to Table 3.

Shut Down

1. Turn off the hot and cold water supplies.
2. Unplug the electrical cord.
3. Open all valves fully and wait until water stops draining from the apparatus.
4. Close all valves fully.

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