Thursday, October 06, 2011

lab report: heat exchanger


OBJECTIVE
The main objective for this experiment is to demonstrate the effect of the flow rate variation on the performance characteristics of a counter-flow and parallel flow concentric tube heat exchanger.

Specific objectives for this experiment include:
  • Learning how the operation of concentric tube heat exchanger. 
  • Developing a set of experiments to obtain statistically significant trends for the overall heat transfer coefficient and the inside heat transfer coefficient as a function of water velocity.
  • Observing the difference between parallel-flow and counter flow operation of the heat exchanger.

INTRODUCTION
Heat exchanger is a device built for efficient heat transfer from one medium to another. A solid wall may separate the media, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. One common example of a heat exchanger is the radiator in a car, in which the heat source, being a hot engine-cooling fluid, water, transfers heat to air flowing through the radiator (i.e. the heat transfer medium).The main purpose of heat exchanger is to remove the heat from the hot fluid and transfer it into the cold fluid. There are 3 types of heat exchanger, parallel flow, counter flow, and cross flow. However, in this experiment, we only consider the counter-flow heat exchanger and parallel flow. Counter flow exists when the two fluids flow in opposite directions. Each of the fluids enters the heat exchanger at opposite ends. Because the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Parallel flow exists when two fluids flow in parallel directions. Each of the fluids enters the heat exchanger at parallel end.

The variables that affect the performance of a heat exchanger are the fluids’ physical properties, the fluids’ mass flow rates, the inlet temperature of the fluids, the physical properties of the heat exchanger materials, the configuration and area of the heat transfer surfaces, the extent of scale or deposits on the heat transfer surfaces, and the ambient conditions. The comparison between counter-flow and parallel flow also can be determined through this experiment which is explained more in discussion part in this report.

THEORETICAL BACKGROUND
One fluid (hot) convectively transfers heat to the tube wall where conduction takes place across the tube to the opposite wall. The heat is then convectively transferred to the second fluid. Because this process takes place over the entire length of the exchanger, the temperature of the fluids as they flow through the exchanger is not generally constant, but varies over the entire length. The rate of heat transfer varies along the length of the exchanger tubes because its value depends upon the temperature difference between the hot and the cold fluid at the point being viewed.

The way that a heat exchanger works is hot water and cold water entering the exchanger, where the process of cold water gaining some heat and the hot water losing some takes place, before they both exit the exchanger. What is actually happening is, the hot water is heating either the inside or the outside of the tubes in the exchanger, depending on where it is flowing, by what is known as convection.

Then the heat is conducted through the tubes to the other side, either the outside or the inside, where it is then converted back into the cold water raising its temperature. Convection is a mode of heat transfer that involves motion of some fluid that either absorbs heat from a source or gives heat to some surrounding. Conduction is a mode of heat transfer in which the heat is moving through a stationary object or fluid. For a heat exchanger that flows parallel or counter current then the coefficient of heat transfer is called the overall coefficient of heat transfer. It is calculated using the log mean temperature difference, which is found two different ways, depending on whether the flow is parallel or counter.

A heat exchanger is a device by which thermal energy is transferred from one fluid to another. The types of heat exchangers to be tested in this experiment are called single-pass, parallel-flow and counter-flow concentric tube heat exchangers. In a parallel-flow heat exchanger, the working fluids flow in the same direction. In the counter flow exchanger, the fluids flow in parallel but opposite directions.

HISTORY
The primary advantage of a concentric configuration, as opposed to a plate or shell and tube heat exchanger, is the simplicity of their design. As such, the insides of both surfaces are easy to clean and maintain, making it ideal for fluids that cause fouling. Additionally, their robust build means that they can withstand high pressure operations. They also produce turbulent conditions at low flow rates, increasing the heat transfer coefficient, and hence the rate of heat transfer. There are significant disadvantages however, the two most noticeable being their high cost in proportion to heat transfer area; and the impractical lengths required for high heat duties. They also suffer from comparatively high heat losses via their large, outer shells.

The simplest form is composed of straight sections of tubing encased within the outer shell, however alternatives such as corrugated or curved tubing conserve space while maximising heat transfer area per unit volume. They can be arranged in series or in parallel depending on the heating requirements. Typically constructed from stainless steel, spacers are inserted to retain concentricity, while the tubes are sealed with O-rings, packing, or welded depending on the operating pressures.

While both co and counter configurations are possible, the countercurrent method is more common. The preference is to pass the hot fluid through the inner tube to reduce heat losses, while the annulus is reserved for the high viscosity stream to limit the pressure drop. Beyond double stream heat exchangers, designs involving triple (or more) streams are common; alternating between hot and cool streams, thus heating/cooling the product from both sides.

EXPERIMENTAL PROCEDURE
  1. This experiment was started by setting the machine for parallel-flow.
  2. The hot water inlet temperature was setted 60oC with decade switch.
  3. The cold water volumetric flow rate ( ... ) was set to run at a constant 2,000 cm3/min.
  4. Initially, the hot water volumetric flow rate ( ... ) was set to 1,000 cm3/min.
  5. This step was repeated for the volumetric flow rate of 2,000, 3,000, and 4,000 cm3/min.
  6. Six temperature reading was recorded in the result table.
  7. Sufficient time (approximately to be 1 to 4 minutes) is allowed, in order to achieve steady conditions.
  8. Step 2 to 7 is repeated for counter-flow heat exchanger operation. 

REFERENCE
  1. Heat and Mass Transfer (A Practical Approach) – 3rd Edition, Yunus A. Cengel, McGraw Hill (2006) 
  2. http://en.wikipedia.org/wiki/Concentric_tube_heat_exchanger
  3. http://www.concentrictubeheatexchanger.com/

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