Carrier 39LA Installation, Start-up And Service Instructions Manual

Also see for 39L Series: Installation, Operation, and Start-Up Instructions

Contents

Carrier 39LA Installation, Start-up And Service Instructions Manual Manual pdf 15 page image

15When a water coil is applied downstream of a direct-expansion (DX) coil, a freeze-up thermostat must be installedbetween the DX and water coil and electrically interlocked toturn off the cooling to prevent freeze-up of the water coil.For outdoor-air application where intermittent chilled watercoil operation is possible, one of the following steps should betaken:• Install an auxiliary blower heater in cabinet to maintainabove-freezing temperature around coil while unit isshut down.• Drain coils and fill with an ethylene glycol solution suit-able for the expected cold weather operation. Shut downthe system and drain coils. See Service section, WinterShutdown, page 30.STEAM COILS — When used for preheating outdoor air inpressure or vacuum systems, an immersion thermostat to con-trol outdoor-air damper and fan motor is recommended. Thiscontrol is actuated when steam supply fails or condensate tem-perature drops below an established level, such as 120 to 150 F.A vacuum breaker should also be used to equalize coil pressurewith the atmosphere when steam supply throttles close. Steamshould not be modulated when outdoor air is below 40 F.On low-pressure and vacuum steam-heating systems, thethermostat may be replaced by a condensate drain with a ther-mal element. This element opens and drains the coil when con-densate temperature drops below 165 F. Note that condensatedrains are limited to 5 psig pressure.INNER DISTRIBUTING TUBE STEAM COILS — Theinner distributing tube (IDT) steam coil used in the Carrier39M air-handling units has an inner tube pierced to facilitatethe distribution of the steam along the tube's length. The outertubes are expanded into plate fins. The completed assemblyincludes the supply and condensate header and side casingswhich are built to slant the fin/tube bundle back toward thecondensate header. The slanting of the assembly ensures thatcondensate will flow toward the drains. This condensate mustbe removed through the return piping to prevent prematurefailure of the coil. The fin/tube bundle is slanted vertically forhorizontal airflow coils, and horizontally for vertical air-flow coils.IDT Steam Coil Piping — The following piping guidelineswill contribute to efficient coil operation and long coil life:1. Use full size coil outlets and return piping to the steamtrap. Do not bush return outlet to the coil. Run full size tothe trap, reduce at the trap.2. Use float and thermostatic (F & T) traps only for conden-sate removal. Trap size selection should be based on thedifference in pressure between the steam supply main andthe condensate return main. It is good practice to select atrap with 3 times the condensate rating of the coil towhich it is connected.3. Use thermostatic traps for venting only.4. Use only 1/2-in., 15-degree swing check valves installedhorizontally, piped open to atmosphere, and located atleast 12 in. above the condensate outlet. Do not use45-degree, vertical lift and ring check valves.5. The supply valve must be sized for the maximum antici-pated steam load.6. Do not drip steam mains into coil sections. Drip them onthe pressure side of the control valve and trap them intothe return main beyond the trap for the coil.7. Do not use a single trap for two or more coils installed inseries. Where two or more coils are installed in a singlebank, in parallel, the use of a single trap is permissible,but only if the load on each coil is equal. Where loads inthe same coil bank vary, best practice is to use a separatetrap for each coil.Variation in load on different coils in the same bank maybe caused by several factors. Two of the most commonare uneven airflow distribution across the coil and stratifi-cation of inlet air across the coil.8. Do not try to lift condensate above the coil return into anoverhead main, or drain into a main under pressure with amodulating or on/off steam control valves. A pumpand receiver should be installed between the coil conden-sate traps and overhead mains and return mains underpressure.9. Use a strainer (3/32-in. mesh) on the steam supply side,as shown in the piping diagrams, to avoid collection ofscale or other foreign matter in the inner tube distributingorifices.NOTE: IDT coils must be installed with the tubes drainingtoward the header end of the coil. Carrier's IDT steam coilsare pitched toward the header end as installed in the unit.10. Ensure the AHU (air-handling unit) is installed level tomaintain the inherent slope. Also ensure the unit is in-stalled high enough to allow the piping to be installed cor-rectly, especially the traps which require long drip legs.11. Do not fail to provide all coils with the proper air vents toeliminate noncondensable gasses.12. Do not support steam piping from the coil units. Bothmains and coil sections should be supported separately.IDT Steam Coil Installation — Refer to drawings to positionthe coils properly with regard to the location of the supply andreturn connections. Ensure that the IDT coil is pitched with thetubes draining toward the header. Carrier’s AHUs provideproper coil pitch when the AHU is installed level.Refer to schematic piping diagrams and piping connectionnotes for the recommended piping methods.NOTES:1. Flange or union is located to facilitate coil removal.2. Flash trap may be used if pressure differential between steamand condensate return exceeds 5 psi.3. When a bypass with control is required.4. Dirt leg may be replaced with a strainer. If so, tee on drop canbe replaced by a reducing ell.5. The petcock is not necessary with a bucket trap or any trapwhich has provision for passing air. The great majority of highor medium pressure returns end in hot wells or deaeratorswhich vent the air.Fig. 18 — Low, Medium orHigh Pressure Coil Piping