AHU Improvement
AHU Improvement
(OP)
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.
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.





RE: AHU Improvement
RE: AHU Improvement
RE: AHU Improvement
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.
RE: AHU Improvement
Anyways, slope the condensate line 1/4" per foot.
RE: AHU Improvement
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.
RE: AHU Improvement
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!
RE: AHU Improvement
You may need to install a new freezstat, those things are known to break.
RE: AHU Improvement
Thanks.