Pumphouse ventilation & cooling

[sunsentinel]

Hi,

There is an unoccupied pumphouse with 1500 hp+ motors in it located in a climate with -40 F winter design conditions. Because there is so much internal heat gain from pumps and piping, this building requires cooling even when it is -40 F outside. Inside design conditions are 95F and 60% RH (a lot of steam in the air)Client asks for a very simple system, easy to maintain. I'm planning to put a 100% outdoor unit with an electric heating coil (no boilers etc acceptable) and temper the discharge air to 58F. This 58F works when outside is -40F but as outside warms up net internal heat gain increases. I would like to reduce the discharge to 42F at this time (no modulation).I have just enough cfm in air handler to start free cooling at 42F. Above 42F we have supply and exhaust fans to help with cooling. We also have unit heaters on their own thermostats to keep the building at 50 F, but I am only imagining them to kick in when 42F supply air starts overcooling the building.
I am worried that 42F supply air will be a bit cold. (less than dew point of 95F/60%RH air.)

Question: Anybody can see anything wrong with this set up?Suggestions?

 

[ChrisConley]

What is summer design? How much air are you moving through the space?

I'd worry about condensation on the ductwork and diffusers.

 

[chicopee]

Where is the humidity coming from? About having movable walls automatically operated by temp. and humidity sensors?

 

[sunsentinel]

Humidity is coming from the process equipment using steam.
summer design is 82F.

 

[HiDrPo]

sunsentinel

Since most motors are rated for 40C continuous operation, I suggest sizing the ventilation CFM rate based on the delta between summer design ambient and 110F. ASHRAE has some guidance for heat gain from motors and hot surfaces. Also include the steam and transmission gains.

Configure your make-up unit with outside and return dampers operated by a space thermostat in the return air path set for 75F and include a motorized relief damper. This provides full air circulation and lets the motors maintain space temperature. You could also consider shutting down the make-up unit when space temperature falls below 55F and leave some natural convection openings to carry away the moisture from steam.

Then leave the existing unit heaters keep the space at 50F.

As long as ambient is below space temperature, I expect it will absorb the space humidity gain (steam) that will then be carried away by the relief air.

If your space includes electronics such as that found in elevator rooms, you may need to maintain 80F in summer which could force you into air conditioning. If so, consider the addition of multiple recirculating DX units with outdoor condensers with capacity temperature that provides a measure of redundancy and set thermostats as high as possible and lock-out the make up unit during cooling.

Good luck and keep us posted.

 

[walkes]

The air handler should have return on it to mix the air for free cooling(depending on your process and code requirements). You can set up the control to maintain space temperature setpoint, modulate the outside air and return dampers to maintain a mixed air temperature.

Heating 100% outside air with an electric coil seems like a waste of money. Verify that you can recirculate the air based on the process, some may require once through ventilation or have a mandated exhaust rate. (Wastewater pump rooms for example)

 

[MintJulep]

I agree that using electric heat to temper -40 outside air seems wasteful, especially since you have such a large source of free heat in the space.

I would:

Install an air cooled air-to-liquid heat exchanger outside and use the free cold to to feed a cooling coil in the return air stream for dehumidification.

Install an air-to-air heat exchanger to recover heat from the exhaust air stream and use it for tempering the supply.

You should never need to consume excess energy for "reheat".

Summer heat removal should be doable with ventilation only from outside air.

Your inside unit heaters should only come on if there is equipment failure to keep the interior above freezing.

 

[sunsentinel]

Mintjulep: Air to air H/X is not possible because exhaust is far from make up air unit. Also, run around coil is not possible because it requires glycol equipment etc.. making things complicated, opposite of client's request for simplicity and no more space for hvac in the building.

walkes: Do you think moist return air mixing with -40F outside air create frost in air handler?

 

[Drazen]

Discussion itself can somewhat help in coming to conclusion like: your problem is not so simple, and very simply solution is unlikely to work. If you could explain it to client, maybe he would allow you to choose effective solution, not necessarily simple one.

Are you familiar with psychrometry? You can find exact state of your air mixture either by manual use for Moliere diagram, or with help of some software.

It is unlikely, however, that you can avoid heavy frosting problem when mixing very humid with very cold air.

 

[walkes]

You may get some frost, your air handler may require summer and winter filter positions to reduce frosting. Try to get good mixing, you should be less than 15% outside air when you are at -40°C

 

[AirSolutions]

I designed three of these systems in Northern Alberta for 1000 HP pumps and are currently designing three more.

We learned a few things along the way and have a good design strategy in place now.

You are on the right track with recirculation.

 

[willard3]

This is not a simple problem and it will not have a simple solution and you will have to tell your client this.

There will be problems with wet-bulb and dry-bulb temperatures.

 

[317069]

I understood the following from your post(please help and correct)
1- unoccupied pump house with existing conditions for winter (95F inside and -40F outside) and for summer of(more than 95F inside, 82F outside)
2- you want to install 100% electrical unit, heat from -40F to 58F, and when the outside temp warmer than -40F you will heat from outside temp to 42F( discharge temp will be either 58 or 42F)
3- you said "I have just enough cfm in air handler to start free cooling at 42F. Above 42F we have supply and exhaust fans to help with cooling. We also have unit heaters on their own thermostats to keep the building at 50 F"
Could you please help to explain this part a little more.
- what do you mean by "building"? is it the pump house itself or other part of builing? I mean are these air handler/ heaters dedicated to serve this pump house only or thay are part of HVAC system that serve a building which includes the pum house as an unoccupied part of it?
4- because the building is an existing one, why the owner needs to install such new simple system now? Also did you have a chance to talke with the original designer?
5- waht is the inside condition in winter and summer that you want to reach by installing a new system?

 

[cry22]

Sunsentinel said"

Air to air H/X is not possible because exhaust is far from make up air unit. Also, run around coil is not possible because it requires glycol equipment etc.. making things complicated, opposite of client's request for simplicity and no more space for hvac in the building.

Sun:
a pump, a coil and glycol for -40F weather is not somethinmg anyone would call rocket science, it is as simple as it can get. There is nothing complicated (or expensive) about use of glycol in -40F weather.

I would caution in use of heat recovery in -40F weather though, you will build frost on the heat exchanger surface, so a pre-heat coil with glycol upstream of HX will be needed for frost control of the HX surface.