TABLE OF CONTENTS So far, we have just assumed that the external heating and cooling requirements are supplied as a single hot utility and single cold utility, at an unspecified temperature sufficient to fulfil the duty. In practice, more than one utility may be available, and there are often price differentials between them. Conversely, the needs of the process may help us to choose the utility levels. The GCC is our key weapon in this area.
Hot utilities, supplying heat to a process, may include:
furnaces,
steam heaters,
flue gas,
heat rejected from heat engines,
thermal fluid or hot oil systems,
exhaust heat from refrigeration systems and heat pump condensers,
electrical heating.
Likewise cold utility systems remove heat from the process. They may include:
cooling water systems,
air coolers,
steam raising and boiler feedwater heating,
chilled water systems,
refrigeration systems and heat pump evaporators,
heat engines below the pinch.
It is also useful to distinguish between constant- and variable-temperature utilities. For example, condensing steam (providing latent heat at a single temperature) is a constant-temperature utility, while hot flue gas (giving up sensible heat over a temperature range) is a variable-temperature utility. Some utilities are a mix of both types; for example, a furnace chamber gives out radiant heat at effectively a constant high temperature, whereas the exhaust gases can release further heat as a variable-temperature utility. The hot and cold utility needs may also be integrated with the power requirements for the site, as...
