STEAM AND HOT-WATER HEATING
Design and installation questions regarding steam and hot-water heating are asked in state licensing examinations at times reference is made to steam and hot-water heating piping systems. Therefore, steam and hot-water heating systems will briefly be discussed.
Steam Heating
Steam is an effective heating medium. It is adaptable to almost any type of building. One of the simplest steam-heating systems is the one-pipe gravity type. Installations of this type are usually limited to moderate- size buildings where the radiators can be positioned at least 24 inches above the water level in the boiler. This type of installation is simple to operate, and the initial cost is low.
Some inherent disadvantages of the one-pipe gravity steam heating systems are as follows:
1. Large piping and radiator valves are required to allow the condensate to return against the resistance offered by the steam flow.
2. Steam and condensate flow in opposite directions with a possibility of water hammer.
3. Air valves are required. Failure of these valves to always open to allow the escape of air may result in slow heat buildup and excessive fuel consumption.
4. Comfortable room temperatures are difficult to maintain unless the radiator valves are regulated by opening and closing. Automatic control of steam from the boiler may result in fluctuating room temperatures.
The two-pipe gravity steam heating system was developed to overcome the difficulty encountered when steam and condensate flow against each other in the same pipe. This system has all the disadvantages (to some degree) listed for the one-pipe system.
Additional disadvantages are as follows:
l. The midsections of the radiators may become air-bound if they are not water-sealed. This happens during the warm-up period when steam fills the radiator nearest the boiler and flows through and enters the return piping.
2. Installation of a valve at each end of all radiators is required so that the steam may be shut off.
If this is not done, steam may be present in both the return and supply lines.
3. The returns from each radiator must be separately connected into a wet return header or water-sealed in some manner.
In a vapor system, thermostatic traps are used at each radiator and at the ends of the steam mains. The radiator inlet valves used in this system are of the graduated or orifice type. The steam pressure necessary is very low: often less than one pound. This type of installation can be used in buildings where 24 inches or more can be provided between the boiler water level and the end of the return line.
Advantages of a vapor steam-heating system include:
1. Air cannot enter-the system as it is closed. Thus, a moderate vacuum is created by the condensation producing steam at lower temperatures.
2. An even, quiet circulation of steam is provided with no air binding or noisy water hammer.
3. Room temperatures can be closely and automatically regulated by thermostatic controls.
4. Radiator air valves are not required.
Disadvantages of a vapor steam-heating system include:
1. Comparatively large pipe sizes are necessary.
2. Only low steam pressure is possible.
3. The condensate must return to the boiler by gravity.
The condensate must return to the boiler by gravity in this type of heating system and may back up in the vertical return pipe when there is excess steam pressure in the boiler. As a result, an air eliminator must be installed well above the level of the water in the boiler, yet low enough for it to close before the return wa- ter is of sufficiently high level to enter the return main. Close control of the boiler pressure is required in this type of system.
A return-trap heating system closely resembles the vapor system except that the return trap provides a positive return of condensate to the boiler. This type can be used in all but the largest buildings if the equivalent direct radiation (EDR) capacity is not greater than the return-trap capacity.
Advantages of a return-trap steam-heating system include the following:
1. The pipe sizes may be smaller because of the higher steam pressures.
2. Return of condensate to the boiler is rapid and positive.
3. The system responds easily to thermostatic control.
4. Steam distribution may be balanced by the use of orifice valves.
There are some disadvantages to the return-trap heating system:
l. Steam circulation depends almost entirely on boiler pressure.
2. Sufficient head room above the boiler must be provided for piping installation.
3. Physical limitations to the size and capacity of return traps exist, which limit the boiler capacity.
If a heating system is limited by the boiler water-level height, by boiler capacity, or because a return trap cannot be used, a condensate-return system may be installed. In this system a condensation pump is installed to return the condensate to the boiler.
Condensate-return systems have the advantage of allowing the return lines to be located below the water level in the boiler and also allowing high steam pressures. However, large drip traps and piping are required for variable-vacuum and vacuum-return-line systems.
Vacuum-return-line systems are similar to condensate-return-type installations except that a vacuum pump is installed to provide a low vacuum in the return line to return the condensate to the boiler.
Advantages of a vacuum-return-line steam heating system include:
1. There is positive return of the condensate to the boiler.
2. Air is removed from the steam mechanically, resulting in a rapid circulation of the steam.
3. Smaller pipe sizes may be used because of the greater pressure differential between the supply and return lines.
The vacuum-air-line system is a variation of the one-pipe steam-heating system. Radiator air vents are replaced with air valves, the outlets of which are connected to a return air line. A vacuum pump is included to exhaust the system of air. Air-line valves are of the thermostatic type.
Advantages of this system include:
l. Steam circulation is more rapid.
2. Radiators heat efficiently at lower pressures.
3. Air vents are not required.
There are some disadvantages:
1. The steam may be noisy at times, as it and condensate flow in the same pipe.
2. Piping and radiator valves must be oversized to accommodate the flow of both steam and condensate.
Two-pipe medium- and high-pressure systems (with ranges of 25 to 125 psi) are used for space heating and for steam-process equipment such as water heaters, dryers, kettles, etc. The two-pipe high-pressure steam heating system is used for space heating and employs tube radiation, fan vector radiation, unit heaters, and fan units with blast coils. High-pressure thermostatic traps or inverted bucket traps are generally used in this type of installation to handle the condensate and to vent the air in the system.
Advantages of two-pipe medium-and high-pressure steam heating systems include:
1. Since the return water can be lifted into the return mains, condensate return lines can be elevated.
2. High-pressure or boiler-feed condensate pumps return the condensate directly to the boiler.
3. Smaller radiators or heat-exchange units and smaller pipe sizes may be used.
4. If the system is used for both heating and steam processing, a simplified piping system may be used with the supply mains being utilized for both the heating and processing equipment. A common condensate system may be installed.