Quantifying Heat Recovery

The following elements are critical to the cost-effectiveness of heat recovery applications:

• The temperature, or quality, of the available rejected heat

• The match between available heat rejection and facility thermal loads

• The capital and operating cost of heat recovery equipment

• The cost of supply alternatives, i.e., the cost of fuel

Quality of Rejected Heat

Combustion engines produce a combination of high-and low-temperature heat outputs. Temperature has a strong relationship to the usefulness of recoverable heat.

The term "quality" is sometimes used to define usefulness along these lines.

High-temperature exhaust gases, typically at 600 to 1,200°F (300 to 650°C), can be used for generating high-pressure steam. Low-temperature heat is available from reciprocating engine water jacket, lubricating oil, valve cage, and charge air cooling systems at temperatures in the range of 100 to 260°F (38 to 127°C). Heat recovery from these systems can be used to generate hot water or low-pressure steam only.

Low-pressure steam from ebullient systems or high-temperature forced circulation cooling systems is limited to 15 psig/250°F (2 bar/121°C). Typical temperatures for hot water raised by engine coolant systems are 160 to 220°F (71 to 104°C), though certain engines are designed to operate at temperatures as high as 260°F (127°C).

Recovered heat from back-pressure or extraction steam turbines can be of varying temperatures and is a function of the specified exhaust pressure. Common outlet pressures are 15 to 50 psig (2 to 4.4 bar), although other pressures between 5 psig (1.4 bar) and 250 psig (18.3 bar) or even higher may be used.

Quantifying Available Rejected Heat

The quantity of available rejected heat can be expressed in terms of the total enthalpy of the rejected heat stream:

Where:

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