Determining Supply Temp W/ High Latent Values

[Polar06]

All,
I am trying to determine the temp of the SA for a warehouse design. I am referencing the Psych chart and I don’t know what to do when the SHR slope does not intersect the saturation curve. The facility has very low internal loads, and the total sensible loads are low when compared to the latent loads. (high latent loads due to ACHs) I have an outdoor temp of 86 degrees F and outside 82% Humidity. Design temp of 75 degrees F and 50% humidity. My SRH is .457, thus I don’t know what to do for a supply temp. Any help is greatly appreciated.

 

[quark]

Calculate the room entry humidity ratio from latent heat formula (you should know the flowrate before this). Draw a horizontal line onto saturation curve and this is your dewpoint. The point where the constant humidity ratio line meets the SHR line is your room entry point. Saturation curve to room entry point is your reheat.

This is one of the cases all HVAC books present. Dedust your book and it will be clear to you.

Also refer faq403-1255

 

[KiwiMace]

Is ACH air changes? Why do you have so many air changes when you are air conditioning? Open doors should be curtained etc.

 

[AbbyNormal]

when you extend the process line and it does not cut the curve because it is steep it means you have an impossible process a coiling coil alone cannot give you the correct dry bulb and moisture content.

double check those loads. If it is correct then you need reheat, you have to over cool the air to the supply dewpoint that you require then you have to reheat the air to the supply dry bulb

It is a symptom of a lot of outside air, or a high concentration of active people

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

 

[AbbyNormal]

besides reheat on the cooling you could have a dedicated dehumidifier, mechanical or desiccant

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

 

[Zesti]

You can see these things as separate items:

-A- figure out which absolute humidity (gr/kg dry air or whatever you Imperial folk use) is needed for the required roomconditions then cool your supply-air to the required level

-B- figure out your cooling load and calculate the supply temperature accordingly

- cool to lowest of the two temperatures (A or B) and reheat to temperature B if necessary.


Note 1: Is the design-condition of 50% RH the maximum allowed value? If it's specified as "50% +/- 10%" then designing for the upper limit of 60% can make a huge difference in energy needed.

Note 2: Do you also need humidification during dry outside conditions?

Note 3: In my experience, people often make mistakes in system-design when requirements are made on RH in the room.

 

[Zesti]

Oh, I forgot about this one:

Is there any stuff stored/brought into in the warehouse that is wet/drying and therefore putting moisture in the air ???

If so, this will influence your calculations/design !!!

Then you would need to determine how much air is needed at which humidity in order to get rid of the excess moisture.

 

[AbbyNormal]

86F @ 82%RH is about an 80F dew point.

I deal with that all the time

So like I was saying, determine the dew point of the air you are supposed to supply. Cool the mixed air down to the dewpoint temperature then reheat it up to the dry bulb you are calculating, OR, use supplimetal dehumidification.

Maybe use an ERV to reduce the latent load of that ventilation air.

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

 

[AbbyNormal]

wondering if Polar is short for Polar Bear Air Conditioning in the Caymans

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

 

[ChrisConley]

That's the funny thing about this forum. I'm not saying we all have to share names and businesses, but it would be nice to have a better idea of who people are.

I've met 'walkes', and he recognized me from this site. Probably wouldn't have happened if I my screen name was HVAC01-1.

I've always assumed that your name is Abby Normal, or that there is an abnormal Abby you're trying to differentiate yourself from...

 

[AbbyNormal]

Ever watch Young Frankenstein?

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

 

[AbbyNormal]

Name like Polar and an 80 dewpoint , the odds are not all that long

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

 

[AbbyNormal]

You should get LinkedIn Chris

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

 

[Polar06]

WOW, thanks for all the info. AbbyNormal you are exactly correct the high latent load is due to outside air. I limited the ACH to 2, and figured I would need a reheat. Are there any good design references for reheats? I am new at this (only 3 designs so far) and I never designed a reheat.

Oh and sorry never been to the Caymons

 

[Zesti]

If it's a warehouse you are talking about, then you probably do not need a lot of fresh outside air to ventilate the building.

If you do have a high coolingload then it would be a waste of energy to let the coolingload determine the amount of outside air: use circulating non-condensing cooling inside the warehouse and only supply enough outside air for ventilation and keeping the RH in the desired range.

Or mix fresh outside air with circulating air from the warehouse at the AHU.


PS: Any Dutch people in here ??

PPS: Being Dutch, not all English/American abreviations and acronyms are familiar to me.

 

[AbbyNormal]

I always get a little suspicious when ventilation is based on so many air changes, but if you are sure you need that much fresh air you could look at reducing the ventilation load with an energy recovery ventilator, maybe it cuts the vent load in half.

You could talk to someone like Engineered Air or (choke cough) Addison and get either an AHU with the reheat or even a 100% outside air unit.

Here is sometyhing I used for a night club-- lots of outside air and large internal latent load. This was custom built by Engineered Air.

An 8 row dx coil with 4 compressors pulled the humid mixed air down to about 52F. The brown is heresite corrosion protection

In the first picture you will see a smaller compressor, its evaporator and condenser are downstream of the main cooling coil. If humidity began to rise this smaller compressor would engage, it would pull down the air temperature 2 more degrees then its heat of rejection would warm the entire air stream up. With the fan and motor heat on top of this reheat it could blow out 65F air that was bone dry.

The reheat can be in many forms, the most sinfuil is electric reheat. I prefer hot gas reheat where it uses condenser heat.

York makes a DR series of roof top units. They are dual circuit. They can be configured to run in what they call the 'alternative reheat mode' Basically on a call to dehumidify, they run both compressors. One compressor will reject its heat outside, the second compressor's heat of rejection reheats the air. You end up with a supply air condition with a similar dry bulb to what enters the unit, just that it is fairly dry. If the space needs temperature control, both compressors dump their heat outside and the unit blows cold air.

http://www.yorkupg.com/NewInfo.asp?id=455

You could also do a moisture balance on the space. You size a cooling system to provide your temperature control. You see how much moisture it will remove. You then look at how much additional moisture needs to be removed and size a dehumidifier accordingly. maybe look at the BKP series dehumidifiers (cha-ching) by heat pipe technology.

http://www.heatpipe.com/

You will be conservative with the above approach.

You can even get the BKP-AC unit which has a remote condenser, they will work like first stage cooling even, blow cold air as long as the space stat requires cooling and then reject the heat outside if the space is cool yet the indoor humidity is high.

Munters is making some dehumidifiers now, desicant based with DX. The condenser heat from the DX is what drives the moisture out of the desiccant wheel.

As others are hinting, this sure sounds like a lot of outside air just for a warehouse.

I am on a small tropical island and see dewpoints like that. I get a litttle suspcious when people from the mainland face humidity like that. I deal with high humidity and the real secret to controlling it is to control how outside air moves inside the building and to precondition it, before the humidity gets into the space. It is like wearing a condom as opposed to going for a shot of penecillin after the fact.

Zesti, I am always amazed how the non-english speaking world still can communicate effectively in English yet when some of us english speakers go abraod we feel that if we talk real slow and loud in english people will understand us.

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

 

[BronYrAur]

How did you determine your SHR?