2000 Hook-up Book

Makeup Air Heating Coils

Figure 44 Air Heater Coils

or sometimes horizontal headers at the bottom of the unit supply vertical finned tubes. The alterna tive arrangement has the headers at opposite sides of the unit, either horizontally at top and bottom or vertically at each side. While each different arrange ment has its own proponents, some general statements can be made, including the fact that even so-called “freeze-proof” coils can freeze if not properly drained of condensate. In “horizontal” coils, the tubes should not be horizontal but should have a slight fall from inlet to outlet so that condensate does not collect in pools but drains naturally. Steam inlets to “horizontal” headers may be at one end or at mid length, but with vertical headers the steam inlet is As steam enters a coil it drives air ahead of it to the drain point, or to a remote area furthest from the inlet. Coil size and shape may prevent a good deal of air from reaching the trap and as steam condenses, a film of air remains reducing heat transfer. Coils with a center inlet connection make it more difficult to ensure that air is pushed from the top tubes, the steam tending to short circuit past these tubes to the condensate header. Automatic air venting of the top condensate header of these coils is essential. With other layouts, an assessment must be made of the most likely part of the unit in which air and noncondens able gases will collect. If this is at the natural condensate drain point, then the trap must have superior air venting capability and a Float-Thermostatic type is the first choice. When an inverted bucket or other type with limited air capacity is used, an auxiliary air vent should be piped in paral lel above the trap. As a general rule, a thermostatic vent and vac uum breaker are desirable on most coils to prevent problems. preferably near the top. Venting Air From Coils

SYSTEM DESIGN

Inlets

Finned Tubes

Alternative Steam Inlet and Condensate Outlet Connecitons

On Coils with Vertical or Horizontal Headers

Outlets

Inlet

Air Vent Location

Inlet

Outlet

Outlet

Inlet

Outlets

Waterlogged Coils The most common cause of prob lems, however, is lack of pressure within the steam space under part load conditions to push conden sate through the traps, especially if it is then to be lifted to a return line at high level or against a back pressure. System steam pres sure lifts condensate, not the trap, and is generally not appreci ated how quickly the pressure within the steam space can be reduced by the action of the con trol valve. When pressure used to push condensate through the traps is lost, the system “stalls” and as condensate backs up into the coil, waterlogging problems of hammering, temperature stratifi cation, corrosion and freeze-up begin. The coil must be fitted with a vacuum breaker so that con densate is able to drain freely to the trap as shown in Fig. II-27 (page 97) and from the trap by gravity to a vented receiver and return pump. This is especially important when incoming air tem perature can fall below freezing. With low coils, this may require the pump to be placed in a pit or lower floor. How to determine

“system stall” conditions and the solution for draining coils to a pressurized return is covered

later in this manual. Vacuum Breaker And Trap Location

A vacuum breaker ensures that some differential pressure can always exist across a trap that drains by gravity but any elevation of condensate after the trap reduces the hydraulic head avail able. Heating is done using an atmospheric air/steam mixture so coil air venting is most important. A vacuum breaker should be fitted to the steam supply pipe, between the temperature control valve and the coil inlet. It is not recommend ed to fit a vacuum breaker on the steam trap where the hydraulic head of water used to push con densate through the trap would hold the vacuum breaker closed. In systems where the return piping is kept under vacuum, a reversed swing check valve should be used and piped to equalize any coil vacuum not to atmosphere, but to the discharge side of the trap.

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