In many respects, there is a significant difference between the physics of steam power systems (such as Rankine cycles) and steam thermal systems. Moving one step further, there is a significant difference in the system design, control, and response characteristics between the steam systems serving high temperature relatively constant load devices such as dryers, cookers, and the like, and those serving the lower temperature variable load devices such as space heating systems. The concept of this difference is touched upon in some of the current activity of second law thermodynamics which recognizes the impact and the value of temperature levels as well as simple energy quantity or flow.

An example of this second law logic is that if one wants to hold the surface of a mill or a dryer at 235 F (129C) , steam at 240 F (116C) (10 psig (68.9 kPa) saturated) may be a perfectly sound medium for accomplishing this. However, if one is attempting to heat air to some temperature which varies between 75 F (24C) and 110 F (43C), steam at 240 F (116C) may, for several reasons, not be the best method to use. This paper will address this latter type system, that of using the latent heat in an intermediate heating fluid to transfer heat from a source to a load, with significant temperature differentials and varying capacity requirements. Specifically, the paper will address those unique features of steam systems (water vapor/water) although it should be kept in mind that most of the physics of two phase thermal systems is not limited in its application to steam. Other fluids could be used; and, in fact, there are considerable advantages to using commercial refrigerants in two phase mode for collecting solar energy.

Assuming an intermediate fluid system is to be used, two phase systems such as steam have several fundamental advantages when compared to single phase systems such as hot water systems. On the other hand, the hot water systems have several advantages over the steam systems. Manufacturers of the devices for these two different types of systems tend to see this advantage-disadvantage listing as a competitive scoreboard when, in reality, this is not the case. In the realm of application engineering the race is over. In the area of terminal control such as standing radiation, fan coil systems, and the like, steam has lost out to the single phase alternative - and for good reason. However, in the initial heat generation components, particularly in larger systems, steam has held its own - and again, for good reason.

A reference to the comparative characteristics of steam and water systems readily reveals why this use pattern has unfolded.

The first comparative characteristic addresses potential reliability. In a steam system, a sudden rupture and resulting loss of fluid at some point in the system will generally not affect the ability of the system to continue to operate. This feature surfaced
when internal combustion engines were used for cogeneration systems. It became evident for example, that a rupture of a flexible pipe connector on an engine jacket connection of one engine did not affect the continued operation of the other engines if they were connected as independent steam generators, whereas, with single phase coolant systems, such a failurewould result in a complete plant shutdown.

The feature of constant temperature heat transfer vs. variable temperature has little significance in affecting the choice of system in most heating system applications. However, in some process applications, the constant temperature has definite advantages.

The "directional" or non directional nature of the distribution system also has little to do with the selection of a system. But lack of recognition of this simple feature has led to extensive errors in piping many large single phase systems.

The characteristic of temperature/pressure dependence has had a significant positive effect upon the use of single phase or water systems over steam. This paper will address that characteristic in some depth.

Of the comparative characteristics listed, the one which has had the most dramatic impact upon the diminishing use of steam as a heating medium is the temperature-pressure dependence. Although there have been rather extensive efforts in the past to achieve stable load control with steam systems, most have failed. This is not simply the observation of the author - it is a fact of the market place! The fundamental purpose of this paper is to study this problem, identify the basic physical causes and explore some new concepts for addressing the solutions.