TWISTED STRIP HEAT EXCHANGER PROJECT REPORT

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ABSTRACT

This is one of the most common problem conventional kind of heat exchangers is that it have an average efficiency and we make a simple study and we found scope to improve efficiency. Our concept is to develop the heat exchanger the heat exchanger which will be a unique laboratory set up for increasing the exact heat transfer rate of the heat exchanger. In this working model, we can easily change the quantity of flowing fluid as well as the direction of flow. This heat exchanger can be easily converted into the parallel flow to cross flow.

INTRODUCTION

Heat exchangers use thin plates or tubes to keep two fluids of different temperatures apart while allowing energy to flow from hot to cold through the wall. The energy transfer acts to change the temperatures of the two media. The hotter one becomes cooler and the colder one becomes hotter. It is required to increase the heat transfer rate for getting more heat transferred thereby to reduce the size and cost of heat exchangers.

Meanwhile, the pressure drop should keep low to minimize the pumping power required. Increasing the Heat transfer with minimum pumping power is known as heat transfer augmentation. Techniques for heat transfer augmentation are relevant to several engineering applications. In recent years, the high cost of energy and material has resulted in an increased effort aimed at producing more efficient heat exchange equipment. Furthermore, sometimes there is a need for miniaturization (highly minimizing the size) of a heat exchanger in specific applications, such as space application, through augmentation of heat transfer.

Furthermore, as a heat exchanger becomes older, the resistance to heat transfer increases owing to fouling or scaling. Minimizing the fouling is also an additional task to be considered in heat transfer augmentation. Fouling is more common for heat exchangers used in marine applications and in chemical industries. In some specific applications, such as heat exchangers dealing with fluids of low thermal conductivity (gases and oils) and desalination plants, there is a need to increase the heat transfer rate. The heat transfer rate can be improved by introducing a disturbance in the fluid flow (breaking the viscous and thermal boundary layers), but in the process pumping power may increase significantly and ultimately the pumping cost becomes high. Therefore, to achieve a desired heat transfer rate in an existing heat exchanger at an economic pumping power, several techniques have been investigated and proposed in recent years by researchers.  

WORKING PRINCIPLE OF TWISTED STRIP HEAT EXCHANGER

In a heat exchanger, the exchange of energy takes place between two fluid which gives its energy to the fluids known as “ hot fluid”. our principle is to provide better thermal contact between the surface and the fluid because secondary flow creates swirl and the resulting mixture of fluid improves the temperature gradient, which ultimately leads to a high heat transfer coefficient. Twisted tape generates a spiral flow along the tube length.

A wire coil insert in a tube flow consists of a helical coiled spring which functions as a non-integral roughness. In a turbulent flow, the dominant thermal resistance is limited to a thin viscous sublayer. The wire coil insert is more effective in a turbulent flow compared with a twisted tape, because wire coil mixes the flow in the viscous sub-layer near the wall quite effectively, whereas a twisted tape cannot properly mix the flow in the viscous sub-layer. For laminar flow, the dominant thermal resistance is limited to a thicker region compared with turbulent flow. Thus, a wire coil insert is not effective in a laminar flow because it cannot mix the bulk flow well, and the reverse is true for a twisted tape insert.

Hence, twisted tapes are generally preferred in laminar flow. Performance and cost are the two major factors that play an important role in the selection of any passive technique for the augmentation of heat transfer. Generally, twisted tape and wire coil inserts are more widely applied and have been preferred to other methods, because the techniques of extended surface inserts suffer from a relatively high cost and the techniques of mesh inserts suffer from a high-pressure drop and fouling problems.  

APPLICATION OF TWISTED STRIP HEAT EXCHANGER

  1. Heating ventilation and refrigeration and air conditioning: louvered fins, corrugated or serrated fins, micro-fins, spirally fluted tubes for boiling and condensing, etc
  2. Automotive industries: radiators, charge air coolers, intercoolers provided with louvered fin, offset strip fin, low fin tubes.
  3. Power sectors: high performance enhanced surfaces on airside developed for dry cooling towers used for fossil power plant heat rejection and turbulators on the tube side of fire tube boilers and structured boiling surfaces in the intermediate heat exchangers.
  4. Process industries: the chemical process industry has cautiously adopted enhancement technology because of concerns about fouling. Wire loop inserts and vapor sphere matrix fluted spheres (tube side), or solid spheres (shell side) not only improve the heat transfer but also reduce fouling with typical process fluids.
  5. Industrial heat recovery: ceramic tubes that are enhanced externally/internally for high-temperature waste heat recovery.
  6. Electronic cooling: enhanced extended surfaces are used for air cooling of electronic devices ranging from radar tubes to microelectronic chips used in computers.
  7. Aerospace: improved gas turbine blade cooling is achieved by transverse repeated ribs and pin fins cast into the blade, thereby reducing the wall temperature.

ADVANTAGES OF TWISTED STRIP HEAT EXCHANGER

The following are the benefits of heat transfer augmentation:  

  1. A decrease in heat transfer surface area, size, and hence decrease in the weight of heat exchanger for a given heat duty and pressure drop.
  2. An increase in heat transfer for a given size, flow rate and pressure drop.
  3. A reduction in pumping power for a given size and heat duty.
  4. A reduction in the approach temperature difference.
  5. An appropriate combination of the above.

DISADVANTAGES OF TWISTED STRIP HEAT EXCHANGER

  1. Initial cost is high.
  2. Mass flow rate cannot be changed beyond a certain limit.

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