Heat Duty

Heat duty is the rate of heat transfer across a defined boundary. It can describe heat added to a reactor, removed by a jacket, supplied by steam, rejected to cooling water, recovered in a heat exchanger, or lost to the environment. The boundary is essential because the same process can have several heat duties at different equipment or utility interfaces.

For a stream with no phase change and approximately constant heat capacity:

where \dot{Q} is heat duty, \dot{m} is mass flow rate, c_p is specific heat capacity, and \Delta T is temperature change. Phase change, reaction heat, heat loss, fouling, and nonideal mixing require additional terms.

Engineering use

Heat duty is used to size heat exchangers, reboilers, condensers, fired heaters, coolers, reactor jackets, chillers, steam systems, hot-oil systems, and heat-recovery networks. It also supports troubleshooting: a change in measured duty can indicate fouling, wrong flow, bad temperature measurement, side reactions, heat loss, or control problems.

Utility capacity should be checked against normal operation, startup, shutdown, fouled conditions, cleaning, abnormal operation, and safe hold requirements. A duty that is adequate at steady state may be insufficient during a transient.

Common mistakes

A common mistake is quoting heat duty without stating the boundary and stream basis. Another is treating calculated duty as exact even though flow, temperature, composition, and heat capacity have uncertainty. A strong heat-duty review states boundary, flow basis, phase behavior, temperature range, heat losses, fouling state, utility conditions, measurement method, and uncertainty.