issue with control valve for LPS reheat operation

Brian O'Sullivan

Have a 100% OA unit with full face (not face and bypass) steam coil with automatic electronic control valve to modulate steam to maintain discharge air temperature for coil. The operation of the valve was not controlled tightly (discharge air range was from 60 deg F to 95 Deg F with a 70 Deg setpoint). The steam is generated from a gas fired boiler with on / off gas control. Boiler cycling between 5-14 PSIG over a 6 minute cycle (2 minute heat up, 2 minute cool down, & 2 minutes to purge and restart). Tech suggest that the steam presure needed to be stabilized with a modulating gas valve setup. By adjusting the gas valve position manually, we were abe to adjust the boiler pressure to about 10 PSIG which seemed allow the coil to work properly. Our opinion is that the control valve should be able to compensate for the 5 -14 PSIG steam pressure range.

If we should control steam pressure more tightly, should we use self piloted PRV or provide modulating gas control for our boiler.

Please comment.

Brian

Brad White_9

Constant Pressure = Better Control

No doubt about it, constant steam pressure is the preferred way to go. We specify pilot operated valves for control valves 2" and up. That is arbitrary and more of a cost concern and all that does for us is give us more instant feedback.

But the core is the valve itself. When you vary the steam pressure that much, your through-put is going to vary by the square. You are asking your steam valve to control to a discharge thermostat while wrestling with the whims of variable incoming pressure.

Most control valves want a pressure drop of about 3-5 PSIG and have enough reserve to get through your traps, maintain coil output.

One reason control valves work well with central/utility steam and not so well when close-coupled to boilers with different agendas.

Face and bypass with a two-stage control valve (1/3-2/3) would be my design intent going in, with the 1/3 valve taking the lead (fully open) and modulating the F&BP damper. But why cast a wish in your face? Not fair, I know....

Go for steam pressure control first then pilot enhancement. That would be my recommendation.


EDIT: Follow-up comment. You do mean pre-heat, not reheat, right? One trick I learned was to use the 1/3-2/3 valve arrangement but the smaller valve might be in actuality 10% of full capacity, nice and tight. That valve would be sized actually for the least load when fully open. Little or no modulation and a 10 to 15 degree rise.

The real application was a one-row preheat coil, wide open to take the edge off, get the air above 35F and allow the downstream coil a tighter control arrangement for final trim. Why not try a tight valve in parallel selected for full open operation? May be better for freeze protection too.

Brad