Run-Around Coil question? 1

[martinakenic]

Hi to all,

I was wonder could anyone explain how works run around coil, I read a lot of about this and I notice that in some system loop we have a three way valve , and some work with two way valve or just with pump frequency regulated. If I correctly understand it well, we use a three way valve in cold climate (if we have a temparature below zero?) to prevent a freezeng coil and also if we want to control temparature supply air, but iam not sure about this, and also Iam not sure when we use two way vale and do we need two way valve at all or is it in this case do we need just pumps freq. regulated.
Do we have some recomendation and standards about this , good praxis case?


Thanks.

 

[GT-EGR]

The 3-way valve is needed anytime you have the ability to bring below freezing temperatures back to your extraction air stream - so if your supply air is cold enough that in winter time the run around loop leaving temperate is below 32, the valve bypases some of the warm side back to your extraction air inlet to mix and maintain a minimum above freezing temp going back to your extraction air stream, otherwise you’ll get frost on that coil

If you have a single extraction coil and a single supply air coil, you can modulate pump speed to control the run around coils heat input to the supply air stream. But if you have multiple supply air streams you’ll need the pump and the 2-way valves.

The bottom diagram is confusing - it looks like it has a 2-way valve but what it describes is that it’s actually an air separator, expansion tank, and safety relief valve.

 

[martinakenic]

Thanks for answer,

and yes bottom diagram have only pump, without control valve ( look like control valve but it isnt), just pump and how in this case we preventing freezing coil on exhaust air coil.

Can I :

1.underdimension AHU Coil on Exhaust Air and on that way preventing that temparature will never fall below zero, or

2.dimension AHU coil on exhaust air in worst winter scenario ( example -21 celsius degrees) and dimensions ahu exhaust coil that I have in those conditions about 4 celsius degree on exhaus air, and I can put regular pump-pump with constant flow, not frequency regulated and thats it about regulating and what will happen with efficiency of the system?

Kind Regards

 

[GT-EGR]

I wouldn't recommend item 1, because the whole point of this run around coil is to save energy. So you'd be reducing your maximum capacity for all hours of operation to protect against the very cold condition scenario. It's also theoretical, so to go with a coil size calculation and have no active coil freeze protection, you would have to be pretty conservative.

Also note the freezing temperature is 32, we are protecting the moisture in the air of the extraction air stream from freezing on the coil. If you have freezing conditions where the run around loop temp can get below freezing, you should protect against that by using a glycol mixture.

To protect against extraction air freezing you will almost always need the 3-way valve, or a double two way valve (one for coil, one for bypass) which essentially does the same thing. In more complicated heat recovery arrangements there are ways to divert your loop back to the extraction air through a heating element, or some process with excess heat, but that's a lot more work than just putting a supply air coil bypass in place.

Theoretically you could have your run around coil flow large enough so it doesn't have a high delta T, but this comes with efficiency penalty since the goal is energy savings, and lower temperature = bigger temperature difference = better heat exchange and more energy savings.

 

[martinakenic]

Hi againg,

I notice on lots of diagram that position of pumps is different:

1.In the ASHRAE diagram, it was to keep constant flow through the exhaust coil and bypass the AHU coil.
2.Coil on the AHU intake have constant flow to prevent glycol gelling.

What is the ASHRAE reasoning on putting the pump flow to run through the exhaust air coil?

Kind Regards

 

[GT-EGR]

I’m not really sure the specifics of why ASHRAE went with their particular arrangement, but I’m sure it is with good reason.

The system likely works with the pump in multiple locations, but I would say it makes the most sense to have it after the extraction air coil since the fluid temperature will always be the least extreme (not very cold and not very warm). I think that matches up with the ashrae diagram you attached.

 

[martinakenic]

Hi,
but take a look this scenario:

When we have ice on exhaust coil , and three way valve close loop on supply AHU unit , in this part of pipe water and glycol mix stay immovable waiting three way valve..but air in supply coil still cool with very cold air because supply ahu still working and blow air into space and glycol starting to gelling...

Kind regards

 

[GT-EGR]

Not sure exactly what you mean by glycol gelling - if your glycol mix is freezing then you may not have enough glycol content relative to your temperatures. Or you have some other chemical in your fluid. Glycol levels need to be maintained.

As for the 3-way valve - you don’t have to get to the point where you have no flow in your supply air, just low enough to keep below freezing, so you can always keep a little bit moving through the coil - just control to a blended temperature, don’t control to full recirculating.

However, even if you did have glycol mix sitting in the supply coil, you should have mixed in enough glycol to bring your freezing temp below your minimum supply air temp.