2000 Hook-up Book

Makeup Air Heating Coils

intermediate temperature but as outside temperature increases, the temperature control lowers the steam pressure in the preheat coil and condensate drainage tends to slow down. If the coil is being used where design loads occur at sub zero temperatures, there can sometimes be only atmospheric pressure in the coil, although the air passing over it is still cold enough to lead to freeze-up problems. This difficulty is greatly reduced if the temperature sensor controlling the steam supply to the preheat coil is set to the needed distribution temperature. Part load conditions would then lead firstly to lowering the steam pressure in the reheat coil, where freezing will not occur, but pressure is maintained in the preheat coil until outside air temperatures are above the dan ger point. Such an arrangement reduces freeze-up problems in many instances on existing instal lations, at minimal cost. Corrosion And Waterhammer Problems Condensate mixed with air becomes corrosive and assuming the boiler water treatment is satis

factory, coil corrosion problems are usually due to condensate regularly backing up or lying stag nant on the bottom of the tubes during shutdown. If the coil is trapped correctly, the most likely cause is an overhead return which prevents the coil from draining. One remedy for this is to fit a liquid expansion steam trap at the lowest piping level, as shown in Fig. II-26 (page 96), set to open when the temperature drops below 90°F. The coil then drains only cold condensate to a sewer. In high pressure systems where waterhammer on startup remains troublesome, a “safety drain” trap is sometimes used. This consists of a stock 15 psi rated inverted bucket trap fitted above the main trap which discharges to drain whenever coil pressure is low, but due to its design locks shut at higher pressure. While this is useful on pressurized mains, the safety trap may require a pressure considerably higher than its nomi nal rating to lock shut and on modulating service a considerable amount of condensate may be wasted. This makes the combina tion pump/trap a more viable solution to this problem.

The steam trap must handle lots of air and drain condensate at saturated steam temperature continuously while the load and pressure are changing and thus a Float-Thermostatic type is recom mended for all air heating coils. The trap is mounted below the condensate outlet from the coil with a vertical drop giving enough hydraulic head to enable a suit able size to be selected. A 14" head should be the minimum and represents about 1/2 psi, a 28" head about 1 psi, and to reduce possibility of freeze-up, a drop of 3 ft. to the trap is recommended. Preheat/Reheat Coils The preheat/reheat coil hookup shown in Fig. II-26 (page 96) may employ a direct-acting temperature control or with larger coils, a quick er responding pilot-operated type with a closer control band is rec ommended. This arrangement allows filtration and perhaps humid ification of the air to be carried out at the controlled preheat tempera ture, and the reheat coil brings the dry bulb temperature of the condi tioned air to the required value for distribution. The preheat coil is used to heat outside air up to the

SYSTEM DESIGN

Case in Action: Air Handling System Steam Coil Drainage

Typical storage buildings are extremely large and difficult to heat. This example in specific has three floors with approximately 486,000 ft 2 of floor space and heated with 150 air handling units. These units are comprised of bay heaters, overhead door heaters and administrative office area heaters. The minimum steam supply pressure to all of them is 20 psig and are pneumatically controlled. In the preceding 12 month period, $201,000 was spent on labor and materials to repair damaged coils. The common problem was condensate standing in the coils, unable to drain, causing erosion due to presence of car bonic acid and bulging/splitting as a result of freezing. Solution Starting with a training session at the facility that addressed this problem and typical solutions, Spirax Sarco’s local sales office implemented a “Cooperative Research and Development Agreement” (CRDA). The purpose of the agreement was to test a proposed solution including Pressure Powered Pumps™ and Pump/Trap combinations to eliminate system stall, thereby assuring thorough con densate drainage, regardless of supply air temperature, control valve turn-down or over-sized heaters. A test was conducted on four air handling units. One unit was hooked up as usual, without Pressure Powered

Pump™ drainage systems. The other three were drained by either open or closed loop PPP systems. Four days into the test and the unit without a PPP drainage system had three frozen coils. It was found that as outside supply air temperature dropped below 36˚F, it was necessary to close outside dampers and use 100% recirculated air, or the coils would freeze. The three units drained by PPP sys tems continued operating trouble-free. Benefits Employee Safety • Improved indoor air quality through the use of a higher percentage of outside air supply. • Reduced chance of injury by eliminating water leakage on the floor from broken coils and subsequent slippage. • Fewer burns because there are fewer steam leaks. • Greater employee awareness of hazards because of training. Cost Savings • Reduced steam and condensate losses resulting in energy savings. • Reduced cost for management support (paper-work). • Cost savings of up to 30% above the initial installation cost in a 12 month period.

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