Question during and job interview and like your opinion

[hollandhvac]

During a telephone job interview I got a build up description of an Air Handling Unit (AHU) and was asked fi this was correct.
The consditioins are as follows: it is a fresh air uint for the arctic area with air in of -35 degrees C and out going of 5 degrees C.

The build up was described as follows:

damper open/close, anti frees unit, heater, drop eliminator, filters (pre and final), humidifier, drop eliminator, fan, damper open/close.


My opinion was that that the filter should go befor the heater and a drop eliminator after the heater was not required.

How ever the interviewer said this was the correct design, but did not tell me why ( to short time ).

What is your opinion of this fresh air unit?

 

[AbbyNormal]

filters can hoarfrost up

as well, if there is snow cover there is not a lot of dirt in the air or flying insects. Not uncommon for make up air units to operate with no filters in the winter

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

Seems ok to me. First reason is given by Abby. Drop eliminators after the heater are for humidifiers (may not be necessary if it is steam humidification).

 

[Zesti]

I would place the fan before the humidifier (no need to bother the fan/electric motor with moist air).

Don't know what grade the final filter is supposed to be, but if it's fairly high-grade then I would place it after the fan (catch grains coming from fan-belts).

But in general, I don't think there is a solution for a specific application which is absolutely the best from every standpoint.

 

[RossABQ]

It was probably more important that you could discuss the merits of any position than to guess the "right" answer. Being able to think is more valuable than memorizing AHU configurations.

 

[hollandhvac]

Do agree with you RossABQ, but like to know why the configuration is the way it is.
Have thought about it over night and now think that the low temperature could make the filter get frozen with ice. Something like the snow particles melt and freeze again that is the rreason for the filter being after the heater where it is plus 5 degrees C. But this is my thought. Like to hear from somebody who worked with it or knows more about it.

 

[SAK9]

The standard confign on 100% fresh air AHUS I have seen is the one described by the interviewer.A frost coil is provided ahead of the filters as ice will build upd on the filter in no time.The frost coil is nothing more than a pipe with 3 or 4 horizontal runs(w/o any fins).The frost coil works full blast with no control valve on them.As quark pointed out already the need for eliminators depend upon method of humidification..But I can not imagine a spray bank humidifier being used in the artic region(in which case eliminator is a must.