Email this pageSteam Heat Transfer
Basic Properties of Steam Heating
When viewed from the standpoint of being a heat medium, steam has superior properties not offered by other heat mediums. Among those, the following two are the most notable:
- Provides even heating
- Provides rapid heating
In this article, we will take a closer look at these properties from the aspect of heat transfer.
How Does Steam Provide Stable, Even Heating?
In the case of saturated steam, if the steam pressure is known then the steam temperature may be determined. Pressure changes instantaneously within a space. When saturated steam condenses, it condenses at the saturation temperature, and the saturated water (condensate) formed is of the same temperature as the saturated steam. This means that if the pressure at the heat transfer surface (the jacket or coil interior of the equipment) is held at a constant, continuous heating will be able to take place at the same temperature at every part of the heat transfer surface.
Speed of Heating
The amount of the heat transfer is indicated by the heat transfer coefficient (= film coefficient of heat transfer).
The units are [W/m² K].
W = J/sec, so if heat exchange takes place on the same heat transfer surface area and with the same temperature difference, the larger the heat transfer rate the shorter the time required for heating.
The rough values for the heat transfer rates of hot water and steam are as follows:
- The rate at which heat is transferred to the heat transfer surface of a heat exchanger using hot water as the heat source:
- 1000 — 6000 [W/m² K]
- The rate at which heat is transferred to the heat transfer surface of a heat exchanger using steam as the heat source:
- 6000 — 15000 [W/m² K]
In actual heating situations, the heat transfer process will be a combination of the mechanism of heat transfer within the walls of the heat exchanger and the mechanism of heat transfer from the wall surface of the heat exchanger to the product being heated. Evaluations of heating must use the overall heat transfer coefficient [W/m² K] to indicate this combination. This coefficient varies greatly from heat exchanger to heat exchanger, but even so, steam heating shows numbers 1.5 – 2 times those of hot water heating.
How Does Steam Provide Rapid Heating?
Heat Transfer from Condensation (Steam)
The secret, of course, is the transfer of heat caused by the process of condensing.
The latent heat contained in steam is released in the instant the steam condenses into liquid phase. The amount of latent heat released is 2 – 5 times greater than the amount of sensible heat in the hot water (saturated water) after condensation. This latent heat is released instantaneously and is transferred by means of a heat exchanger to the product being heated.
Heat Transfer by Convection (Hot Water and Oil)
In contrast, hot water and oil are used in convective heating, which does not involve a phase change. Instead, the heat medium reduces its own temperature in order to transfer heat to the product being heated. A mainstream in industry is the use of forced-convection by means of equipment such as a pump to create the flow against the heat transfer surface.
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