AHU Improvement

[JCar123]

We have an AHU consisting of 2 steam coils, air filters and a fan supplying warm air into a lab. The steam supply is 2 bar.
It sits outside.
The original design is such that it trips out at -5 C (supply air).
We have been having colder winters which means that the system trips out more often than we would like. If it trips out in the night or over a weekend and if it is off for long enough, the condensate line freezes. This causes us a lot of disruption.
Please can you help me to understand why we have a trip at -5 C and what is the best option for improvement.
Many thanks in advance.

 

[MechEngNCPE]

Do you mean -5C ambient outside air conditions? You may need to check your freezestat after the preheat coil. Or the preheat coil is not sized to adequitely prevent the cooling coil from freezing at such low ambients, so the unit shuts itself off. You may need to replace the preheat coil and freezestat with one of sufficient capacity that will prevent your cooling coil from freezing well below -5C.

 

[MechEngNCPE]

Sorry, no chilled water coil, I assumed there was one and just read too quickly. Anyway, is there a direct digital controller that can allow you to modify the cut-out temp setpoint? Or some other way to change that setpoint? One would have to see a p&id of the unit to tell you what temp sensor trips the power.

 

[JCar123]

We have been able to reduce the trip set point temporarily which I believe is the supply air (ambient, outside) temperature but what impact would this have on the system? Is it acceptable? How was -5 C derived?
I have noticed that the steam condensate lines are not sloped so therefore I can understand why we have been getting blockages.
We would really like a simple solution like changing the trip set point to a lower value to prevent the unit tripping out as long as the long term impact on the system is understood and acceptable.

 

[MechEngNCPE]

Your supply air temp setpoint is different from the Outside Air temp setpoint for de-energizing the unit. Is this unit on a BMS? If so, reset the OA lower limit cut-off to a lower temp. I'm sure it is adjustable.

Anyways, slope the condensate line 1/4" per foot.

 

[ChrisConley]

Is the -5C an 'auto-low limit'?

Are the steam coils sized for the outdoor conditions you've described?

What controls the steam coils? Indoor temp, or discharge temp?

You mentioned 2 steam coils, is one preheat? Is there temperature sensing between the coils?

Ensuring that you aren't lifting the condensate is a very good first step. Modulating steam coils can have freezing problems depending on the rate of the coil.

You could consider a strap-on aquastat on the condensate line, have it open the steam valve and turn off the fan if the condensate temperature drops below 35F or so.

 

[JCar123]

The unit consists of fresh air entering an isolation damper before it reaches the first coil, labelled as the frost coil. There is a sensor downstream of this coil and it is set at 15 C. The steam control valve opens and closes to meet this setpoint. It then enters 2 filters in series before it reaches the "re-heat coil" and it appears that the fan discharge temperature is controlled by the steam control valve on the second coil. The lab temperatures (4 labs) have temperature sensors and the average is taken.

I am not sure where the temp sensor is that trips out the whole unit. I can find out. As the unit sits at a high level outside and takes fresh air from outside I imagine there is not much difference between the outside air temp and the supply temp.

The idea about switching off the fan when the condensate temp is low sounds good however we need to keep the balance of air right within the labs which has an extract system (fume cupboards).

As far as the -5C limit is, will this be to protect the coils, or simply the limit on the system to be able to achieve the desired temperatures within the lab.

I have trouble with some of the acronyms mentioned in the previous emails. Please elaborate.

Many thanks for your responses!

 

[MechEngNCPE]

"As the unit sits at a high level outside and takes fresh air from outside I imagine there is not much difference between the outside air temp and the supply temp." ...wat? The outside air (OA) temp is certainly much lower that the supply air (SA) temp. One is freezing and the other has been heated.

You may need to install a new freezstat, those things are known to break.

 

[JCar123]

Apologies, when I think of supply air, I think of air supplied to the AHU unit not the air supplied by the unit.
Thanks.