Reading Materials: Chapter 9 Heat Transfer Heat transfer results from a temperature difference. LECTURE 25

Heat Exchanger Devices A heat exchanger is a device through which a hot fluid (liquid or gas) flows in intimate contact with a cold fluid so that heat can be exchanged from the hot to the cold fluid.

Heat Exchanger Devices A hot stream is a process stream that needs to be cooled. An example is a refinery crude oil stream that needs to be cooled from 150°C to 100°C. A cold stream is a process stream that needs to be heated. An example is milk that must be heated from 5°C to 85°C to pasteurise it. A hot utility is a source of heat that is separate from the process streams. It is usually steam. A cold utility is a sink of heat that is separate from the process streams. It is usually cooling water or ambient air.

Heat Exchanger Devices

Heat Exchanger Devices A co-current or parallel-flow heat exchanger is one in which the hot fluid always flows in the same direction as the cold fluid. This flow arrangement requires the cold fluid leaving temperature to be less than the hot fluid leaving temperature and results in a lower temperature difference compared to counter-current flow.

Heat Exchanger Devices A counter-current or counter-flow heat exchanger is one in which the hot fluid always flows in the opposite direction to the cold fluid. This flow arrangement gives the highest possible temperature difference over the whole exchanger and therefore results in the smallest heat exchanger for a given duty. It also permits the cold fluid leaving temperature to be higher than the hot fluid leaving temperature.

Heat Exchanger Devices In this course, heat exchangers are always counter-current to best utilise the temperature difference between the hot and cold streams.

Energy Balance around H/E Sensible heating and cooling:

Energy Balance around H/E Phase change: Chemical reaction:

Example A heavy oil stream must be heated to a higher temperature, so the decision is made to use a heat exchanger with saturated steam being condensed to saturated water as the heating source on the other side of the exchanger. The characteristics of the oil are: Oil mass flow rate: 960 lbm/min Oil mean heat capacity: 0.74 Btu/lbmoF Oil inlet temperature: 35oF Desired oil outlet temperature: 110oF The saturated steam has the following properties: Steam temperature: 280oF Heat of vaporization (@280oF): 925 Btu/lbm What steam flow rate is needed for this exchanger?

Solution Hot Cold Balance on cold stream: Saturated Steam, 280oF, mstream Saturated water, 280oF, mstream Hot Cold Oil, 35oF, 960 lbm/min Oil, 110oF, 960 lbm/min Balance on cold stream:

Solution Balance on hot stream: