Heat transfer fundamentals (5 of 5)

Connections

Connections form an important part of any heat exchanger as they provide the interface between the unit and the system pipework. It is essential for the safe and convenient working of the equipment that the type of connection is chosen carefully to satisfy the safety requirements for the design pressure and temperature and the clients requirements for convenience and suitability for the process. If a heat exchanger is going to be removed for cleaning on a regular basis then quick release clamps may be a more appropriate choice than bolted flanges.

It must be appreciated however that both flanges and the various styles of quick release clamps offered by suppliers are limited in working pressure and temperature. It is essential that whichever style of connection is specified by the end user (or offered by the designer) the temperature and pressure ratings of the connection are checked to confirm acceptability.

In addition to pressure and temperature rating, the process compatibility of the gasket or seal material used for the connection must always be checked. There are a range of materials available for most connection types and if the equipment is to meet the customers’ requirements the one chosen must be appropriate for the working fluids. It must also be appreciated that gasket materials differ not only in chemical composition and therefore process compatibility but also differ in Hardness and Thickness which must be matched to the requirements of the flanges. If the material is too hard on a smooth faced flange it may not seal properly while if it is too soft on a high pressure flange it may not retain its correct position. Gasket materials and thicknesses must be checked with a specialist manufacturer to ensure compatibility with the application.

As important as the type of connection used is the size of the connection. This will often be specified by the installer but the heat exchanger designer must be aware of the implications of the sizes chosen. The latest revision of the HED thermal design programme includes a dialogue box which allows the designer to check the fluid velocities, values of ρv² and nozzle pressure losses when choosing or checking nozzle sizes. It is essential that these values are always checked as an incorrect choice could affect the functionality of the equipment.

Mechanical considerations also affect the choice of nozzle. Essentially the bigger the hole in the shell the weaker the shell will become and under pressure vessel rules the weakening effect must be checked and if necessary reinforcement added in the nozzle area. On the HRS K series units the nozzle sizes chosen as standard are normally the maximum that can be fitted onto the shell. Whatever size is chosen however the effects must be checked for pressure safety.

Bolted flanges normally carry a standard pressure rating based on a working temperature of 100°C and above this temperature the acceptable design pressure is reduced.

Type of Fitting	Maximum Design Pressure	Maximum Design Temperature	Comments
PN6 Flange DIN 2573	6,0 Barg @ 100°C	Refer to DIN 2573	Gasket material must be specified
PN10 Flange DIN 2576	10,0 Barg @ 100°C	Refer to DIN 2576	Gasket material must be specified
PN16 Flange DIN 2502	16,0 Barg @ 100°C	Refer to DIN 2502	Gasket material must be specified
RJT	8,0 Barg	Depending on seal material	Seal material must be specified
IDF	8,0 Barg	Depending on seal material	Seal material must be specified
Tri-clamp	8,0 Barg	Depending on seal material	Seal material must be specified
DIN 11851	8,0 Barg	Depending on seal material	Seal material must be specified
SMS	8,0 Barg	Depending on seal material	Seal material must be specified