CONTROL VALVE APPLICATIONS TEMPERATURE CONTROL of an AIR HEATING COIL using a Temperature Control Valve
PURPOSE:
For accurate control of the temperature of air when the benefits of a control valve are required. The air may be heated through a single coil or bank of multiple coils referred to as an Air Handling Unit (AHU).
OPERATION:
A temperature control valve offers response times and characteristics suitable for continuous heating applications with wide variations in flow demand. The temperature sensor (RTD or Thermocouple) senses the temperature of the liquid being heated and allows modulation of the valve for appropriate supply of steam through the complete control loop. Pneumatic or modulating electric actuators may be used.
INSTALLATION GUIDELINES:
(see Figure17)
• Care should be given to the installation of the temperature sensor to ensure full immersion in the liquid. The sensor should be placed as close as possible to the heater tank to ensure accurate temperature control of air. • When the AHU is located near the ground, the steam trap from each coil is recommended to have inline connections in order to limit height and maximize the ability to drain by gravity. It is recommended to have a separate steam trap on each individual coil. • Eccentric reducers, if required, are used on valve inlets to prevent accumulation of condensate which could become entrained with high-velocity steam, possibly resulting in dangerous waterhammer. • While a separator is appropriate for protection of the Regulator, it is not always required, as a properly sized drip leg with steam trap may be sufficient. It is recommended for systems where steam is known to be “wet” and the entrained moisture could affect valve performance and/or result in component damage. • Consider low-cracking pressure (1/4 PSI opening pressure) check valves after steam traps when discharging intocondensate return lines. Check valves eliminate the possibility of condensate backing up through the steam trap into the system. • A vacuum breaker and auxiliary air vent should be properly installed to promote proper drainage and optimum heat t transfer. The vacuum breaker allows system equalization with atmospheric air to allow gravity condensate drainage when vacuum is formed from condensing steam. The air vent improves heat-up times and overall heat transfer by expelling accumulated air on start-up. • A safety relief valve (SRV, not shown) is appropriate where applicable codes dictate their requirement, or anywhere protection of downstream piping and equipment from over-pressurization is desired. Consult the factory for appropriate SRV sizing guidelines.
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