unit heater control - 3way or run wild?

[scha0786]

I am installing three new unit heaters. The last unit heater is maybe 80' from the 8" mains.

I want to install a 3-way control valve on the last unit heater to keep some flow.

I have a controls guy in my office that wants to just run the glycol wild and cycle the unit heater fan for control.

I am totally against this but need some fire power to come back with. so far:

-3-way provides the flow, same as running wild
-no control valve will always reject heat, even in the summer

 

[walkes]

hot glycol in the summer? not enough info on the existing system. We have done both ways, depends on the system.

 

[scha0786]

yes, this is a oil process facility in alaska. Glycol runs (200F) 24/7 365 a year.

 

[scha0786]

I was confused....

he wants all five unit heaters with no control valves, just let the glycol run all year round and cycle the fans on and off. That would be 130 GPM running wild thru coils all year long.

for sure crazy, right?

 

[Drazen]

it looks like improvisation, but when you mentioned conditions you are in, that somewhat changes the picture.

for some time i was working in siberia that likely has similar climatic conditions, and there is big issue of having enough flow through units most of time. even with highest feasible glycol concentration you still have danger for freezing much of year, and enabling flow mitigates the risk.

i would say zoning is the main thing that should affect decision. if your new heaters belong to the same zone already served by other heaters, of if that zone has exactly the same usage schedule like others on the same loop, you could go without control valves.

 

[scha0786]

Yeah no control valves is actually not sounding like such a bad idea now, lol

god I hate it when I'm wrong

 

[walkes]

No control valves also reduces the pressure drop.

 

[mtngreen]

In addition, if the system pumps are running off VFD's the lack of control valves on the UH's can provide the minimum flow requirements of the pumps.

 

[scha0786]

NO VFD's on the pumps.

 

[SNORGY]

Nothing wrong with circulating hot water / glycol 24/7, other than it costs money, since the pumps run continuously and there will inevitably be glycol losses that otherwise might not be there. The three-way bypass valve wouldn't change any of that.

Summer case heat rejection equipment (presumably extraction air fans) will now have additional load to shed, and as the Operator wipes beads of sweat off his brow and opens the doors, cursing the engineer for running the hot water all year around, he will eventually get frustrated and close the glycol supply valve(s) to the heater(s).

In anticipation of that scenario, my vote would beblock valves on the inlet and outlet plus a manual globe valve on the heater bypass. It gives 100% continuous flow capability and bypasses the heat rejection area associated with the heater, imparting marginal, if measurable at all, incremental heat rejection duty on the extraction air fans in the summer.

Regards,

SNORGY.

 

[317069]

- no VFD on pumps this tell that it is a constant voleum system
- 3 way valve provides constan voleum flow, then the 130 gpm is the same in both cases with or without control valves.
- you want to install 3 way to keep some flow runing, how much do you want to keep?
- runing wilde will reject heat even in summer as you said, but waht generate heat in summer, I mean if you have boiler what make this boiler works in summer?
- fans usaully need more energy than pumps in HVAC application
- what kind of piping system do you have.?

 

[scha0786]

Its a very complicated heating system.

They have turbine compressors that have heat recovery for the glycol loop. These turbines run 24/7 365 and provide a constant heat source for the glycol heating (200 170).

The pumps are constant volume and also run 24/7 365.

The 3 new unit heaters I am installing are Trane 200mbh vertical glycol. The new unit heaters will be on a new loop from an 8" glycol main. The unit heater loop is around 150' total round trip.

I wanted to install 2-way control valves on the two unit heaters and put a 3-way on the last (end of the loop) unit heater to keep some flow down at the end.

Each unit heater is fed from above and will have isolation valves, low point drain, high point vent, circuit balancing valves and flex connectors at each connection.

The piping spec up here calls for 100% welded pipe and flange connections, no threaded connections!.

3" and up is sch 40 cl 150
2" is sch 80 cl 150
1-1/2" and below is sch 160 cl 150

yeah overkill for glycol but thats spec.

 

[317069]

- your system description sound it is designed for year around heating demand, if this is the case why are you afraid from going without control valve
- heat comming from compressors has to be rejected somewher (in you case it is the glycol loop) if you stop rejection where this heat would go?
- are these 3 new unit serve one space or 3 separate spaces
- are they ceiling mounted type
- do existing units have control valves?
- your unit give 200MBH when ther is a certain cfm air passing the coil and if the cfm has changed (unit's fan is on or off) the heat rejected from coil will change.

 

[walkes]

Where do you reject the heat when the building doesn't need it if the turbine compressors are running 24/7? Do you have a radiator or other heat rejection device?

 

[Drazen]

as walked said, there is no way your system can run without parallel system of heat rejection, and that system certainly has to be sized to be able to reject all heat from compressor, even if it is needed for one single hot summer day in a year.

that way, you certainly have to have that as a solution on how to avoid undesired heat rejection during summer, on system level