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— Piping Practices —
1. Steam unit heaters condense steam rapidly, especially during warm-up periods. The return piping must be
planned to keep the heat-exchanger’s core free of condensate during periods of maximum heat output, and
steam piping must be able to carry a full supply of steam to the unit heater to take the place of condensed
steam. Adequate pipe size is especially important when a unit heater fan is operated under on-off control
because the condensate rate fluctuates rapidly.
2. Heater is to be connected and serviced only by qualified personnel experienced in piping work. For
additional piping information refer to industrial piping handbooks and related literature.
3. Eliminate pipe stress by adequately supporting all piping. Do not rely on heater to support piping.
4. Take off all branch lines from the top of steam mains, preferably at a 45° angle, although vertical 90°
connections are acceptable.
5. Pipe the branch supply line into the steam unit heater’s inlet at the top and the return branch line from the
outlet at the bottom.
6. In steam systems, the branch from the supply main to the heater must pitch down towards the main and be
connected to its top in order to prevent condensate in the main from draining through the heater, where it
might reduce capacity and cause noise. In long branch lines, a drip trap may be needed.
7. Allow for pipe expansion to prevent excessive strain on the unit heater’s heat-exchanger core.
8. The return piping from steam unit heaters should provide a minimum drop of 10 in (254 mm) below the
heater so that the pressure of water required to overcome resistances of check valves, traps, and strainers
will not cause condensate to remain in the heater.
9. In steam systems, where horizontal piping must be reduced in size, use eccentric reducers that permit the
continuance of uniform pitch along the bottom of piping (in downward pitched systems). Avoid using
concentric reducers on horizontal piping, because they can cause water hammer.
10. Installing dirt pockets at the outlet of unit heaters and strainers with 0.063 in. (2 mm) perforations to prevent
rapid plugging are essential to trap dirt and scale that might affect the operation of check valves and traps.
Strainers should always be installed in the steam supply line if the heater is valve controlled.
11. In steam systems, rapid air removal is required because entrained air is a cause of corrosion. Proper air
venting for steam systems can be achieved by use of a steam trap with an internal air vent.
12. Steam traps must be located below the outlet of the unit heater. Consult the trap manufacturer for specific
recommendations. Each steam unit heater should be provided with a trap of sufficient size and capacity to
pass a minimum of twice the normal amount of condensation released by the unit at the minimum differen-
tial pressure in the system. Trap capacity is based on the pressure differential between supply and return
mains. Steam systems should be equipped with a float and thermostatic trap or inverted bucket trap with an
air bypass.
13. If the condensate return line is above the heater outlet or is pressurized, install a check valve after the
steam trap and a drain valve at the strainer to drain the system during the off season.
14. Install pipe unions and shut-off valves to and from each unit heater to allow maintenance or replacement of
unit without shutting down and draining the entire system.
15. Adequate air venting is required for low-pressure closed gravity systems. The vertical pipe connection to
the air vent should be at least 3/4 in. NPT to allow water to separate from the air passing to the vent. If ther-
mostatic instead of float-and-thermostatic traps are used in vacuum systems, a cooling leg must be
installed ahead of the trap.
16. In high-pressure systems, it is customary to continuously vent the air through a petcock unless the steam
trap has a provision for venting air. Most high-pressure return mains terminate in flash tanks that are
vented to the atmosphere. When possible, pressure reducing valves should be installed to permit operation
of the heaters at low pressure. Steam traps must be suitable for the operating pressure encountered.
17. On steam systems where the steam supply to the unit heater is modulated or controlled by a motorized
valve, a vacuum breaker should be installed between the unit outlet and a float and thermostatic trap.
