Minimum Temperature requirement for keeping the pipes and equipment freezing

[Mehtesham]

Hello Guys,

I just need a little help. We have two big Air houses each has a capacity of 240,000 cfm, and these are serving the areas which are abandoned, they only have machinery and steam and other condensate pipes. We also have a few small air houses which are near to these two big giant ones. We are thinking of abandoning the small ones in place and use the air/energy of the big two to serve the other areas. I was wondering what is the minimum temperature requirement to keep the pipes and equipment from freezing (Is it 55F?), so that we can divert the other available cfm to use at some place else. Should I use 1 cfm/sq ft for my calculation. I am confused need suggestion. Thanks

 

[DRWeig]

Hi Mehtesham,

Well, water freezes at 32°F, so anything above that will work. You must take into account air distribution and zoning -- if there are windows with sprinkler piping or other water lines nearby, without adequate circulation you may have a problem Add some degree of safety factor or else monitor the temperature around the perimeter of the space and at the roof to be sure you don't get a freeze-up in a cold spot. Also consider the deadband or proportional band of your temperature controls and whether your heating source has adequate capacity.

In a situation like you describe (abandoned area with sprinkler or other piping), I have seen 35°F (risky, in my opinion) up to 40°F (the most common, in my experience). I have also seen a much more energy-efficient installation, in which all pipes subject to freezing are heat-traced and the building is not heated at all. For safety, the operating status of the heat trace can be monitored as well.

Wait and see what others have to say.

Good on ya,

Goober Dave

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[KiwiMace]

If you are heating a space then you need to work out the heat load to understand what cfm to supply. 1 cfm/sf sounds like an outdoor air quantity for ventilation. You might be able to glean the heat load from bms data if you have it - pick a nasty winter day and see what was happening at the AHU/s.

 

[Mehtesham]

There is not heat load in that area since its abandoned. Only pipes running through that and some conveyors over there. I am thinking of using 0.5-0.75 cfm/sq ft only for that abandoned area and use the rest of the cfm to deliver somewhere else. That other place is occupied and it has people in there, So I would use the the same supply temperature which is existing now and only deliver less cfm to the abandoned area. Does that sound right? More suggestion please.

 

[Shophound]

What temperature do you want to keep the abandoned areas at?

Say you pick 40 degrees to give you a buffer against freezing any pipes running through the abandoned section. From there you can figure how many CFM are required at your winter outdoor design day conditions to hold that space at 40 degrees.

If you work at the facility full time you can experiment/adjust the airflow into those spaces and carefully monitor them with zone temperature sensors. If you have a BMS, tag the zone sensors with alarms to notify staff in case the space temperature dips below 40 degrees. BMS are powerful tools for projects like this; so many times the abilities of BMS are underutilized.

 

[IRstuff]

You pick a temperature, figure out what the heat loss from those areas would be for that temperature, and supply the required amount of heat through your airflow. It seems to be a fairly deterministic problem.

TTFN
faq731-376

 

[IRstuff]

Conversely, if you have controllable airflow, you simply plop a thermostat into those areas, set the desire temp and let the thermostat do its thing.

TTFN
faq731-376

 

[urgross]

It would be helpful to know the type of HVAC configuration you have in the abandoned and occupied space. Is all heating directly from AHU discharge air, does the abandoned areas have reheats and existing boxes? Is the occupied area preheat/cooling coil/reheat? Are the systems VAV or CV?

As the area is abandoned hence no minimum air flow requirements, and you have steam available, why not just steam trace all water piping as suggested by DrWeig?

 

[KiwiMace]

If you feel you need to heat it, then there is a heat load.