Value of Steam as a Heat Transfer Medium

Steam usage is generally expressed, in English units, in lbm or thousand lbm (Mlbm) and, in SI units, in kg. Rudimentary steam balances are quantified in these terms and steam is typically sold in these quantities. However, as can be seen by steam tables and thermodynamic analysis, steam only has value based on what can be done with it. A lbm (or kg) of steam at 0 gauge pressure may have a negative value. Even though it still has heat value, it often cannot be used productively and, therefore, must be con-

productively at a facility and must be discarded. It may also provide unwanted heat in a facility, which then requires work either to exhaust or to cool air or equipment.

The variable cost of steam is primarily a function of fuel cost, boiler efficiency and the difference in enthalpy between incoming feedwater temperatures and exiting steam conditions. There are also a series of ancillary costs for steam, such as makeup water, water treatment, maintenance, blowdown losses, feedwater and condensate pumping, and combustion air supply.

densed and eliminated.

A more useful way to express the value of steam, however, is based on its Btu (kJ) content. Energy accounting systems often refer to energy usage in terms of dollars per million Btu (or kJ) of steam or million Btu (or kJ) of steam per unit of production. From the steam tables, it can be seen that a lbm (kg) of saturated steam at 250 psig (18.3 bar), which has a saturation temperature of 406°F (208°C) and an enthalpy of 1,202 Btu/lbm (2,795 kJ/kg), has more value than a lbm (kg) of saturated steam at 25 psig (2.7 bar), which has a saturation temperature of 267°F (131°C) and an enthalpy of 1,170 Btu/lbm (2,722 kJ/kg). As a heat transfer media, the difference in value is more than the proportionality of the total enthalpy, which is only a few percent — 32 Btu/lbm (74 kJ/kg). This is because the saturation temperature of the higher pressure steam is about one and one half times greater — 139°F (59°C) — than the lower pressure steam.

When heating a fluid entering at 150°F (66°C), for example, the difference in saturation temperatures versus the temperature of the fluid to be heated is more than double — 256°F vs. 127°F (124°C vs. 53°C). The lower pressure steam can provide the same amount of heat to the fluid, but a much larger heat exchanger will be required. The maximum fluid temperature attainable is limited to some temperature lower than the saturation temperature of the steam.

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