Since the days of the Roman Empire, hot water and hot gases have been circulated as heating media. Some time around 1920, a new concept in industrial and process heating was first successfully tested in Europe. In 1927, the first technical journals were written in the United States concerning high temperature liquid and hot water systems. Newer, stronger materials were developed, pressures and temperatures were increased, and by 1930, many 400°F (204°C) circulating systems were in operation in England.

These systems began to grow in the post-World War II days. Since then, these systems have operated extremely well, and most owners will attest to their satisfaction. Numerous systems have followed, proving that high temperature hot fluid for process heating is successful.

Advantages of Hot Water Boiler Systems

These systems were adopted and continue to remain popular choices for industrial process heating applications for a number of reasons. They include:

  • Low initial equipment cost.
  • Low operating cost.
  • Long equipment life.
  • Applicability to variable elevation installations.

Lower Initial Equipment Cost. The comparative cost of the high temperature hot water heating equipment is less than the same capacity in steam.

Lower Operating Cost. The maintenance and operating costs of hot water boiler systems can result in cost savings. The savings result from several features:

  • Lower fuel expense due to higher operating efficiency and less system radiation losses.
  • Lack of steam trap discharge losses.
  • Virtually no feedwater makeup required.
  • Reduced costs due to labor savings by the elimination of water softening, blowdowns and boiler water treatment. Boiler attendants normally are not required. The system can remain in operation without supervision and can be started with a time clock.

Long Equipment Life. Because little or no water is induced into the closed system, corrosion or scale problems are kept to a minimum. These systems typically run for long periods of time with little maintenance.

Applicability to Variable Elevation Installations.Because the circulating system depends on a pump for circulation, it is not important that the pipe be graded in order for condensate to reach a low point. The distribution piping system is easy to install with fewer complications than steam systems.

System Components for Hot Water Boiler Systems

It is important to understand the components of a system and how each operates as part of the whole design. The main components include:

  • Boiler.
  • Expansion tank.
  • Circulating pumps.
  • Air separator.
  • Heat exchanger.

Let’s take a closer look at each system component.

Boiler. There are many boiler manufacturers that make Section IV hot water boilers. The limitation of this design and code is 250°F (121°C) and 160 psi.

There are a few flexible-tube and firetube manufacturers that can build and trim above these limits by building hot water boilers to the Section I, or the power boiler code. For example, some manufacturers can furnish units to 300 psi and 400°F (204°C). This enables higher temperatures using reduced heat exchanger size while still providing good heat transfer, which is more efficient.

Expansion/Compression Tank. Some suitable means must be supplied in order to accommodate the expansion of water volume within the system when the system is heated to normal operating temperature. Specially designed ASME-code tanks are necessary for this application and should be rated to withstand system pressure and temperature.

A wide range of tanks is available. The importance of purchasing the correct size and design of expansion/compression tank cannot be minimized. In some instances, little design consideration is given to the expansion tank. This can result in pump problems or discharging of the boiler relief valve, thus losing treated water.

In some instances, compressed air or nitrogen can be used to control system pressure and expansion. Water glass fixtures are standard on expansion/compression tanks and tank-mounted, low-water cutoffs are available at extra cost. Some manufacturers are able to supply tanks rated for temperatures up to 450°F (232°C) that can be built for pressures up to 250 psi.

High Temperature Circulating Pumps. Of utmost importance to the entire satisfactory operating system is the circulating pump. Each job demands a specific pumping capacity (in gallons per minute) and a specific maximum head capacity. When an inadequately sized pump is furnished, the boiler tubing may become starved for water.

Be sure your manufacturer can select and supply air- or water-cooled pumps to ensure adequate system flow. Economical air-cooled pumps can be provided for system operation to 275°F (135°C). Air-cooled pumps are available for temperatures to 325°F (163°C) and pumps with water cooling jackets are available to up to 400°F (204°C).

Air Separator. A suitable air separator should be provided and able to withstand system pressure and temperatures.

Heat Exchanger. A suitable means of transferring the heat must be provided and designed for use with circulating fluid. If the exchanger is not properly sized, the heat cannot be transferred successfully to the product and the system will not work efficiently.      

Hot Water System Definitions and Examples

Low Temperature/Low Pressure. Also known as LT/LP, low temperature is defined as those below 250°F and low pressure is defined as less than 160 psig.

Medium Temperature/Medium Pressure. Also known as MT/MP, medium Temperature ranges from 250 to 350°F (121 to 177°C) and less than 150 psig.

High Temperature/High Pressure. Also known as HT/HP, high temperature is defined as those above 350°F and high pressure is defined as less than 300 psig.

Closed System. One in which the fluid is continuously circulated with no water makeup or lost fluid.

Open System. One in which the fluid is heated and then used, so makeup is required.

Some examples of hot water heating systems include the following.

  • A domestic hot water system with heater, pump and storage tank constitute LT/LP open system. In this use, the water would normally be in the 140°F range with a pressure of 100 psi or less.
  • A closed water system on plating tanks where the circulating water is less than 350°F, and approximately 115 psi, would be a MT/MP system.
  • A 400°F (204°C) circulating thermal liquid heater or hot water boiler operating on a jacketed kettle, platen or calendar rolls would be an example of a HT/HP system.