Steam Heating Systems Part 1: The Properties of Steam

By Mark Bingham

Steam has been used for comfort and process heating for over a century. While it offers numerous advantages, a thorough understanding of its properties is essential for a successful application. In this series, we discuss steam properties and how they apply to steam distribution, heating processes, and condensate return systems.

While superheat occurs transiently in some steam processes in HVAC systems, this article will focus on saturated steam, which is most common in HVAC applications. We start with the properties shown on an abbreviated saturated steam table, shown in Table 1. (A glossary of the terms contained within this table is provided at the end of this post.)

Table 1

The first column in the steam table is the pressure. For saturated steam, the pressure fixes or “sets” the other properties. For example, at zero psig (atmospheric pressure at sea level), it is well known that water boils at 212°F. This value, which represents the "saturated temperature" at zero psig, is shown in the second column from the left.

The third column is the specific enthalpy of saturated liquid. Water that is boiling at atmospheric pressure has an energy content of 180 Btu/lb. At 32°F, water has a specific enthalpy of 0 Btu/pound·°F. Since the specific heat of water is 1 Btu/ pound·°F., heating water from 32°F to 212°F requires the addition of 180 Btus/pound. To evaporate a pound of water that we’ve heated to 212°F, we must add 970 Btus (fourth column), bringing the total energy content to 1150 BTUs/pound (fifth column). Over five times as much energy is required to evaporate a pound of water as was needed to bring it to a boil.

The high energy content of steam is one of the properties that makes it attractive as a heat transfer medium. However, it also demands thoughtful risk mitigation. Uninsulated surfaces can become dangerously hot, and safety measures and vigilance must be observed to prevent burns due to steam escaping from leaks or opened valves.

The last column in our steam table is the specific volume of saturated vapor (steam). While our abbreviated table omits the specific volume of water, it is 0.0168 ft3/lb at atmospheric pressure. The corresponding specific volume of steam is 26.8 ft³/lb. This is almost 1600 times the volume of water. The significant volume difference requires consideration when steam is condensed in heating processes.

In future blogs, we will discuss the implications of these steam properties in heating applications.