Heat Convection

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Abstract

A double pipe heat exchanger is examined by experimental methods for single-phase flow. The fluid is water at atmospheric pressure. Temperature measurement is made throughout the set point 9 for H951 and set point 6 for H950 system. Mass flow rate of hot and cold water was control by the nob. Heat is supplied to the inner tube stream by an involvement heater. The heat transfer coefficient was calculated using the stander correlations. Both systems were in turbulence flow the entire time. The understanding with expectations is very good for the counter flow arrangement, but not very good for the parallel flow arrangement.

Table of Contents 

INTRODUCTION AND THEORY (20%)……………………………………………..        1

PROCEDURE AND METHODS (10%)……………………………………………….        2

RESULTS AND DISCUSSIONS (40%)……………………………………………….        3

CONCLUSIONS (5%)…………...……………………………………………………...   4

REFERENCES (5%)...…………………………………………………………………...  5

QUESTIONS (10%)……………………………………………………………………..        6

Introduction and Theory

Temperature can be defined as the amount of energy that an element/ substance has. To transfer the heat from one substance to another heat exchanger are being used. It is very important to control the temperature of incoming or outgoing steam or hot liquid, in process unit. Heat exchanger main job is to raise or lower the temperature of these steams or liquid by transferring heat to or from the steam.

Heat exchangers are a device. Its job is to exchange the heat between two fluids of different temperatures that are separated by a solid wall.  One of the simplest types of heat exchangers is the double pipe heat exchanger.  The reason it is called a double pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first pipe. In the double heat exchanger flow can be co-current or counter-current. There are two flow configurations; one of them is co-current, it is when the flow of the two streams is in the same direction, and the second is counter current flow, in this the flow of the streams in opposite directions.

The convective heat transfer coefficient  is defined according to Newton's Law of Cooling as[pic 6]

[pic 7]

Where q is the surface heat transfer rate,  the wall temperature,  the surface area and the bulk fluid temperature.  The value of  directed by operating parameters and as well as physical properties of the fluid. [pic 8][pic 9][pic 10][pic 11]

Transfer heat from one working fluid to another fluid is called Heat exchangers. For example steam generators, feed water heater, re-heater and condensers are examples of heat exchangers found in nuclear power systems. The rate of heat transfer across a heat exchanger is usually expressed in

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Where:   = heat transfer rate [pic 13]

U = overall heat transfer coefficient

A = heat exchanger area

= Average temperature difference between the fluids [pic 14]

PROCEDURE AND METHODS

• Connect the heat exchanger unit H950 and H951 for counter-current flow.
• Now, Fully open the high flow water control valve.
• Switch on the main power switch. Then set the heater input to maximum and raise the hot water temperature to about 68 C. (In the lab manual it says 70 C)
• Adjust the cold-water flow rate to bring the mean hot water temperature to about 68 C.
• Now wait till system reaches steady state.
• Record inner and outer tube temperatures and flow rates.
• Now, Reduce the hot water flow rate to about 90% of maximum.
• Do not change the cold-water flow rate. Therefore use the heater control to bring the mean hot water temperature back to its original value.
• Repeat this procedure for hot water flow rates of approximately 10 % increments of the initial value
• Allow conditions to stabilize and repeat the measurements
• Turn off the system and make flow to Concurrent flow and repeat each step.

Result and Discussion

Equations:

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H950 and H950x:

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H950:

  - [pic 20]

Temperature difference across tube side of heat exchanger

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Heat transfer rate for tube side of HX

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 Heat transfer rate for shell side of HX

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 Logarithmic mean temperature difference

H951: