High Temperature Water Heating

High-temperature water systems are classified as those operating with supply water temperatures above 350°F and designed to a pressure rating of 300 psig. The usual practical temperature limit is about 450°F because of pressure limitations on pipe fittings, equipment, and accessories. The rapid
pressure rise that occurs as the temperature rises above 450°F increases cost because components rated for higher pressures are required . The design principles for both medium temperature and high-temperature systems are basically the same. In this chapter, HTW refers to both systems.

The following characteristics distinguish HTW systems from steam distribution or low-temperature water systems:

• The system is a completely closed circuit with supply and return mains maintained under pressure. There are no losses from flashing, and heat that is not used in the terminal heat transfer equipment is returned to the HTW generator. Tight systems have minimal corrosion.

• Mechanical equipment that does not control the performance of individual terminal units is concentrated at the central station.

• Piping can slope up or down or run at a variety of elevations to suit the terrain and the architectural and structural requirements without provision for trapping at each low point. This may reduce the amount of excavation required and eliminate drip points and return pumps required with steam.
• Greater temperature drops are used and less water is circulated than in low-temperature water systems.

• The pressure in any part of the system must always be well above the pressure corresponding to the temperature at saturation in the system to prevent flashing of the water into steam.
• Terminal units requiring different water temperatures can be served at their required temperatures by regulating the flow of water, modulating the water supply temperature, placing some units in series, and using heat exchangers or other methods.

• The high heat content of the water in the HTW circuit acts as a thermal flywheel, evening out fluctuations in the load. The heat storage capacity can be further increased by adding heat storage tanks or by increasing the temperature in the return mains during periods of light load.

• The high heat content of the heat carrier makes high-temperature water unsuitable for two-pipe dual-temperature (hot and chilled water) applications and for intermittent operation if rapid start-up and shutdown are desired, unless the system is designed for minimum water volume and is operated with rapid response controls.
• Higher engineering skills are required to design a HTW system that is simple, yet safer and more convenient to operate than are required to design a comparable steam or low-temperature water system.

• HTW system design requires careful attention to basic laws of chemistry and physics as these systems are less forgiving than standard hydronic systems.

Elements of High-Temperature Water System
BASIC SYSTEM

High-temperature water systems are similar to conventional forced hot water heating systems. They require a heat source (which
can be a direct-fired HTW generator, a steam boiler, or an open or closed heat exchanger) to heat the water. The expansion of the heated water is usually taken up in an expansion vessel, which simultaneously pressurizes the system. Heat transport depends on circulating pumps. The distribution system is closed, comprising supply and return pipes under the same basic pressure. Heat emission at the terminal unit is indirect by heat transfer through heat transfer surfaces. The basic system is shown in Figure 2.

The principal differences of HTW systems from low-temperature water systems are the higher pressure, heavier equipment, generally smaller pipe sizes, and manner in which water pressure is maintained.

Most systems are either (1) a saturated steam cushion system, in which the high-temperature water develops its own pressure, or (2) a gas- or pump-pressurized system, in which the pressure is imposed externally.

HTW generators and all auxiliaries (such as water makeup and feed equipment, pressure tanks, and circulating pumps) are usually located in a central station. Cascade HTW generators sometimes use an existing steam distribution system and are installed remote from the central plant.

DESIGN CONSIDERATIONS

Selection of the system pressure, supply temperature, temperature drop, type of HTW generator, and pressurization method are the most important initial design considerations. The following are some of the determining factors:

• Type of load (space heating and/or process). Load fluctuations during a 24-h period and a 1-year period. Process loads might require water at a given minimum supply temperature continuously, while space heating can permit temperature modulation as
a function of outdoor temperature or other climatic influences

. • Terminal unit temperature requirements.
• Distance between heating plant and space or process requiring heat.
• Quantity and pressure of steam used for power equipment in the central plant.
• Elevation variations within the system and the effect of basic pressure distribution. Usually, distribution piping is the major investment in an HTW
system. A distribution system with the widest temperature spread
(Dt) between supply and return will have the lowest initial and operating costs. Economical designs have a Dt of 150°F or higher.

The requirements of terminal equipment or user systems determine the system selected. For example, if the users are 10 psig steam generators, the return temperatures would be 250°F. A 300 psig rated system operated at 400°F would be selected to serve the load. In another example, where the primary system serves predominantly 140 to 180°F hot water heating systems, an HTW system that operates at 325°F could be selected. The supply temperature is reduced by blending with 140°F return water to the desired 180°F hot water supply temperature in a direct-connected hot water secondary system. This highly economical design has a 140°F return temperature in the primary water system and a Dt of 185°F.

Because the danger of water hammer is always present when the pressure drops to the point at which pressurized hot water flashes to steam, the primary HTW system should be designed with steel valves and fittings of 150 psi. The secondary water, which operates below 212°F and is not subject to flashing and water hammer, can be designed for 125 psi and standard HVAC equipment.

Theoretically, water temperatures up to about 350°F can be provided using equipment suitable for 125 psi. But in practice, unless push-pull pumping is used, maximum water temperatures are limited by the system design, pump pressures, and elevation characteristics to values between 300 and 325°F.