HEAT TRANSFER - PRINCIPLES & EQUIPMENT - FACTORS AFFECTING HEAT TRANSFER PDF Print E-mail
Written by Norrie   
Wednesday, 17 February 2010 20:47
Article Index
HEAT TRANSFER - PRINCIPLES & EQUIPMENT
HEAT TRANSFER BY CONVECTION
BASIC HEAT TRANSFER CALCULATIONS
LINEAR AND CUBICAL EXPANSION
HEAT EXCHANGERS, BOILERS & FURNACES
TYPES OF HEAT EXCHANGER
EXCHANGER SHELL SIDE FLOW PATTERNS
EXCHANGER TUBE SIDE FLOW PATTERNS
FACTORS AFFECTING HEAT TRANSFER
All Pages

FACTORS AFFECTING HEAT TRANSFER

  1. Temperature Difference (.T ) between the two fluids - This is the driving force in heat exchange principles. The greater the .T, the greater the heat transfer rate .
  2. Fluid flow rate - Increasing flow rate will increase heat transfer rate.
  3. The nature of the heat conducting materials - Some materials have a high conductivity while others don't. This factor is 'built-in' in the design of the Exchanger and choice of materials. It is governed by the design engineers before manufacture.
  4. Surface area - The larger the surface area of the conducting interfaces, the greater the heat transfer rate.

The surface area, again, is controlled by the design and manufacture of the exchanger. The more tubes contained in the bundle, the greater the surface area. The tube length will also affect heat transfer, as will the outside diameter and metal thickness of the tubes.

The main problems encountered with heat exchangers are as follows: -

  1. Fouling: - This is caused by deposits of scale, dirt, sand and / or other solid deposits on the conducting surfaces. Coke formation in furnace tubes and other causes of semi-blockage of tubes will drastically decrease efficiency in an exchanger. Such problems will result in shutdown for cleaning and possible tube replacements. Many of these problems can be avoided by proper operation and fluid treatment - filtration, corrosion inhibition, furnace firing control .. etc.
  2. Air pockets: - The formation of air pockets in exchanges due to improper venting at start up, or build of gas from light materials, will affect the heat transfer rate. This can be avoided by venting all air or gas out at start up and periodically venting gases as required.
  3. Leakage: -Most leakages occur due to gasket failure - replacement of gaskets will be necessary. Internal leakage due to tube failure will cause contamination of the lower pressure fluid by the ingress of higher pressure fluid. This will require tube 'plugging' or replacement. Tube failure generally occurs due to corrosion, excessive pressure or by failure of the welded or rolled fitting of the tubes into the tube-sheets.

 

About the Author

Norrie is a retired professional who has been working in Oil and Gas and LNG production in Marsa-el-Brega, Libya for 30 years.

Norrie used to be in the Training Dept. and prepared Programmes for Libyan Traine



Last Updated on Wednesday, 24 February 2010 19:40