de-humidification with a DX coil 1

[mechanicaldup]

I have an application where the outside design condition is 35°C, 75 % RH.
The room design condition is 22°C, 50 %RH. A relative small amount of outside air is supplied to the room (infiltration)

How do you indicate on the psychrometric cart the de-humidification process? How do you calculate the expected %RH in the room?

thks

 

[KenRad]

Are you sure about the 35C and 75%RH occurring simultaneously? Those are some severe conditions!

---KenRad

 

[imok2]

Outside air
Air at 95*f and 75%RH = 189.3 gr moisture/lb
enthalpy = 52,64btu/lb
SpV = 14,59 cu ft/lb
Room air
Air at 72*F and 50% RH = 58.38 gr moisture/lb
enthalpy = 26.4btu/lb
SpV = 13.58 cu ft/lb
per lb means weight of air moved
so if you remove the difference of moisture and heat between the two you should get the results you are looking for. Of course this doesn't account for infiltration.

 

[quark]

The link below gives you an excellent description of various psychrometric processes and the calculation procedures. Go to the Technical Zone.

http://www.coolit.co.za/psychart/

Regards,

 

[mechanicaldup]

thks guys for your advice.
Will the de-humidification be possible with a DX-coil?

 

[mechanicaldup]

Quark

What psychrometric software could you recommend?

 

[AEBuck]

May I jump in and say "Heatcraft" Professional Edition at handsdownsoftware.com.

 

[mechanicaldup]

How would you represent the above mentioned cooling process on a psychrometric chart, how would you connect the 3 points?

 

[lilliput1]

Bring in outdoor air through the AHU and pressurize the space to avoid infiltration. If infiltration is at entrance door, provide heated only vestibule. Read up Carrier Airconditioning Handbook to learn psychrometry. You will not get far with HVAC if you don't understand it.

 

[ChasBean1]

mechanicaldup -

The psych chart can give good beginning-to-end conditions. Don't rely on your ruler lines to tell you what really happens inbetween. Cool: go straight, horizontal left until you hit the dew point (100% RH) line. Then follow the dew point line down until you hit the right leaving dry bulb temperature (vertical).

A DX coil or chilled water coil can dehumidify, BUT the DX poses more risk in that it cycles off during part load conditions. During off cycles, warm and humid air is brought in until cool surfaces that the cooling air produces sweat and cause condensation.

If DX is needed, look for multiple stage DX and hot gas bypass. One phenomenon about the psych chart is that it doesn't account for cooling coil bypass. Your resulting point isn't actually saturated, but close. Cooling coil bypass allows more dehumidification via a phenomena too difficult to explain here. As a result, typical coils will discharge 55°F air at about 90% (versus 100%) RH.

-CB

 

[mechanicaldup]

Thks Chasbean

the thing is your room condition/ R/A changes from actual outside conditions to the actual design room condition as your initial Room condition aproximate thoe outside condition.

In other words: how do you indicate on the chart your initial point of 80% RH that must be dehumified to 50%RH, how do you represent this process line?

 

[quark]

This is not simply plotting two points on a psychrometric chart and then adding them together. What you asked in one sentence is all about HVAC. Briefly, first calculate the sensible and latent loads in your control space. These loads include the sensible and latent loads by the infiltrated air also.

Mark the outside air condition and the required room condition on the psychrometric chart. Now get the ratio of sensible heat to the total heat(sensible heat + latent heat). You can find a protractor with sensible heat ratio slopes marked on it. Replicate the same line on the chart originating from the room condition and towards the saturation line. Now connect the ambient condition and the point where SHR line meets the saturation line(Actually this is a short cut to the process of extending a horizontal line from outside condition to saturation and then dropping it down along the saturation line to the required point)

Your actual room entry point depends upon the coil bypass factor. I suggest you to either take membership of ASHRAE(you will get one handbook free per year + one monthly journal free) for USD 150 or purchase the Handbooks. Other good reference is Principles of Air Conditioning by Shan K Wang.

You can download a good psychrometric calculator from

http://pmilligan.net/pmtherm.htm

but this will not help you in load calculations.

Regards,

 

[ChasBean1]

Quark, I agree. I think what our friend mechanicaldup is looking for is a process in a single space between time t=0 and time t=x.

Mechanicaldup, the beginning to end I was referring to really meant an outside air condition to a steady state indoor air condition. You have a complex question. Best engineering practices available to non-Ph.D. types deal with steady state conditions at point A and point B, i.e., outdoor and indoor or return air. The lines you draw on the psych chart are between location A and location B, but NOT location B to location B ten minutes later. This is more complex. However, you can represent the before and after conditions on the psych chart by two different points, e.g., a space that started at 77°F, 55% RH and ended at 74°F, 52% RH. You can look at the two points and figure out the difference in anything (enthalpy, dew point, wet bulb, etc.) but the line between doesn't mean much of anything as far as rate of change, etc...

Does this make sense regarding your question, or am I off track?

 

[lilliput1]

You need to calculate the sensible and latent cooling load. You have to determine the CFM and conditions (dry bulb temperature and humidity) leaving the cooling coil such that sensible and latent loads are met. Sensibly, that is when the supply air is heated by the fan heat, duct heat gain, room sensible loads (lights, people, equipment, building envelope, infiltration, etc.)the supply air temperature will rise up to the indoor design value. Latentwize it means that when the latent loads in the space (people, equipment latent, infiltration, etc.) is absorbed by the supply air, its humidity will rise to the acceptable design value range (say 45 to 50% RH). As initial approximation you can use 55°F cooling coil leaving temperature but higher loads may require down to 50°F. However you must make sure the temperature & humidity leaving the coil is just below saturation, not saturated or not in the fog region.
Then you have to calculate the air entering the cooling coil conditions. You have to do mixed air calculations of the outdoor air with return air. You have to know their CFM. Use the outdoor design condition for the OA. Use room design condition plus return duct heat gain plus return air fan heat gain for the return air conditions.
The cooling coil duty is the the sensible and latent heat difference between coil entering and leaving conditions.
If you can calculate the above, it is a cinch plotting the state points on the psycrometric chart. The tricky part is making sure the cooling coil leaving condition is just below the saturation line.
See Carrier Airconditioning Design Handbook for the pschrometric formulas involved.

 

[mechanicaldup]

Thks Gents

like ChasBean1 have noted, I am concerned about the transient from the start condition at ambient conditions to the final room condition. how to determine if equipment x would be able to dehumidify the space and wheter a de-humidifier is required in serie. and to determine in what time the space would be dehumidified. how do I handle this?

 

[mechanicaldup]

any advice on how to determine the "de-humidification capacity" of a DX-coil?

 

[ChasBean1]

mechanicaldup, you MAY (or I may) be confused regarding the end you're actually looking for. Could you clarify?

 

[mechanicaldup]

ChasBean1

I am trying to figure out how determine the de-humidification effect.

for example for Room A
start condition
t1=35
RH1= 80%

after a certain time period
t2= 22°C
RH2=?

 

[cme]

dx coils come in a variety of configurations ..... face split, row split and intertwined ..... the last is the best ..... if possible, shut the OSA damper, go full cooling and kick in reheat

what kind/make of air handler is this?, cfm? tonage?

 

[mechanicaldup]

do you think I shall contact manufacturers of of dehumidification equipment in this regard?

Wo is large suppliers of de-humidification equipment?