ASHRAE Journal - September 2009 - 55

ASHRAE 19 a great improvement over the messy, inefficient stoves and fireplaces people had been using. Low pressure steam systems are still used for heating today, and they provide an easy way for engineers to understand: • Some characteristics of steam; • Steam system operation; • Some problems that have developed in the use of steam; and • The design ideas and system components that have been developed to solve those problems. 50 59–2009 YE A R S JOURNAL As air is vented, pressure in the steam main drops, promoting more steam flow from the boiler. Steam pressure also drops due to friction as it flows through the system’s pipes and fittings. After steam enters a radiator, it gives up the latent heat it absorbed in the boiler and condenses to form a liquid called condensate. The great reduction in volume as steam condenses also acts to promote steam flow through the system. Latent heat at constant saturation temperature is released in the condensing process to travel through the radiator walls and warm the room. Condensate from the radiators flows under the influence of gravity back to the boiler to repeat the cycle. All of the piping in early steam systems was pitched toward the boiler to allow condensate to return. This counter flow piping meant that the steam and condensate were flowing in opposite directions in the same pipe at the same time. Steam Characteristics A steam boiler contains just enough water to establish the level recommended by the boiler manufacturer. Before the burner starts to fire, the boiler volume above the water contains air, as does the rest of the system piping and the radiators located in each of the rooms to be heated. When the burner fires, heat transfers to the water, raising its temperature. There is a definite, predictable relationship between the amount of heat added to the water and the resulting rise in water temperature up to the boiling point. However, at the boiling temperature, the continued addition of heat changes the liquid to steam with no further increase in temperature. Heat that is added in raising water temperature to the boiling point is called sensible heat, because we can sense the effect of the heat addition by observing the rise in water temperature. Heat that is added at the boiling temperature is called latent heat, because it only changes the liquid water to a vapor with no change in temperature. There is no fundamental difference between sensible and latent heat; September 2009 those words simply describe the effect of the heat as it is transferred to the water. Current terminology for sensible heat is liquid enthalpy; and for latent heat, enthalpy of evaporation. Specific enthalpy refers to the amount of heat carried in one pound of the fluid. Water expands a little as it is heated, but it expands a lot as it changes from liquid to vapor. At 0 psig (0 kPa), it is a volume ratio of about 1:1600. This increase in steam volume within the confined space above the liquid leads to an increase in boiler pressure. The higher pressure in the boiler, as compared to the lower pressure in the system, causes steam to flow. No pumps or blowers are required to distribute steam through the system. Pressure and temperature in the typical heating system boiler are locked together at what is called saturation conditions. A rise in saturation pressure is always accompanied by an increase in the boiling, or saturation, temperatures as described in the steam tables.1 Values for specific enthalpy and specific volume of the steam and liquid at different conditions of temperature and pressure are also found in the steam tables. Low Pressure Boilers By definition, a low pressure steam boiler operates below 15 psig (104 kPa). Most heating systems are designed to operate well below that limit, usually at 1.5 or 2 psig (10 or 14 kPa). Cast iron construction is common in these boilers, although other materials are used too. Boiler pressure is maintained within acceptable limits by a combustion system that controls fuel and air flow to the burner to generate steam at a rate that will match the system’s demands. The combustion control systems on these boilers are simple and inexpensive. They automatically cycle the burner on and off, or from high fire to low fire (higher and lower fuel ASHRAE Journal 55 Steam System Operation Air must be vented from the system as steam pressure builds because steam cannot enter the piping or radiators if they are blocked with air (air bound). Air venting is also important in maintaining a high heat transfer rate because the temperature of an air/ steam mixture will always be less than the temperature of pure steam for a given pressure.

ASHRAE Journal - September 2009

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