ANALYSIS ON THE EFFECT OF TUBE INSTALLATION ON THE PERFORMANCE OF DOUBLE CROSS FLOW HEAT EXCHANGER PROJECT REPORT

INTRODUCTION TO HEAT EXCHANGER

A heat exchanger is a device used to transfer heat between a solid object and a fluid, or between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.

1.2 CLASSIFICATION OF HEAT EXCHANGERS
There are three primary classifications of heat exchangers according to their flow arrangement.

1.2.1 THE COUNTER FLOW HEAT EXCHANGER
A counter flow heat exchanger has the hot fluid entering at one end of the heat exchanger flow path and cold fluid entering at the other end of the flow path. Counter flow is the most common type of liquid-liquid heat exchanger, because it is the most efficient. A double pipe heat exchanger is usually operated as a counter flow heat exchanger. The flow pattern in a shell and tube heat exchanger with a single tube pass will be approximately counter flow if it is long in comparison with its diameter. Because of the baffles and the need to distribute the flow of the shell side fluid over the cross-section of the shell, the flow is not as close to counter flow in a shell and tube heat exchanger as it is in a double pipe heat exchanger.

1.2.2 THE PARALLEL FLOW HEAT EXCHANGER
A double pipe heat exchanger can be operated in parallel flow mode. Similarly, a shell and tube heat exchanger can be operated in approximately parallel flow by having both fluids enter at one end and exit at the other end. With parallel flow the temperature difference between the two fluids is large at the entrance end, but it becomes small at the exit end as the two fluid temperatures approach each other. The overall measure of heat transfer driving force, the logarithmic mean temperature difference is greater for counter flow so the heat exchanger surface area requirement will be larger than for a counter flow heat exchanger with the same inlet and outlet temperatures for the hot and the cold fluid.

1.2.3. THE CROSS FLOW HEAT EXCHANGER
A car radiator and an air conditioner evaporator coil are examples of cross flow heat exchangers. In both cases evaporator coil heat transfer is taking place between a liquid flowing inside a tube or tubes and air flowing past the tubes. With a car radiator, the hot water in the tubes is being cooled by air flowing through the radiator between the tubes. With an air conditioner evaporator coil, air flowing past the evaporator coils is cooled by the cold refrigerant flowing inside the tubes of the coil. Cross flow heat exchangers are typically used for heat transfer between a gas and a liquid as in these two examples.

1.3. TYPES OF HEAT EXCHANGERS

1.3.1. DOUBLE PIPE HEAT EXCHANGERS
Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate. However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same.

1.3.2. SHELL AND TUBE HEAT EXCHANGER
Shell and tube heat exchangers consist of series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C). This is because the shell and tube heat exchangers are robust due to their shape.

Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums through holes in tube sheets. The tubes may be straight or bent in the shape of a U, called U-tubes.

1.3.3. PLATE HEAT EXCHANGER
Another type of heat exchanger is the plate heat exchanger. These exchangers are composed of many thin, slightly separated plates that have very large surface areas and small fluid flow passages for heat transfer. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasket type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently bonded plate heat exchangers, such as dip-brazed, vacuum-brazed, and welded plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with "chevron", dimpled, or other patterns, where others may have machined fins and/or grooves.

When compared to shell and tube exchangers, the stacked-plate arrangement typically has lower volume and cost. Another difference between the two is that plate exchangers typically serve low to medium pressure fluids, compared to medium and high pressures of shell and tube. A third and important difference is that plate exchangers employ more countercurrent flow rather than cross current flow, which allows lower approach temperature differences, high temperature changes, and increased efficiencies.

1.3.4. PLATE AND SHELL HEAT EXCHANGER
A third type of heat exchanger is a plate and shell heat exchanger, which combines plate heat exchanger with shell and tube heat exchanger technologies. The heart of the heat exchanger contains a fully welded circular plate pack made by pressing and cutting round plates and welding them together. Nozzles carry flow in and out of the platepack (the 'Plate side' flowpath). The fully welded platepack is assembled into an outer shell that creates a second flowpath (the 'Shell side'). Plate and shell technology offers high heat transfer, high pressure, high operating temperature, fouling and close approach temperature. In particular, it does completely without gaskets, which provides security against leakage at high pressures and temperatures.