DUCT HEATER SIZING FOR MAINTAN RH

[moideen]

Not the sizing of heaters for heating the room, but rather the sizing of heaters to maintain the RH in the duct, is the subject of this post. As far as we are aware, the duct heater sizing equation is CFM*DT/3160. Using this, the air in the room is heated in accordance with the heat load. However, the necessary delta T is crucial to decreasing the rh level in order to preserve the supply air's relative humidity increasing from the saturated condition. For a 400-cfm unit, for instance, the ideal supply air temperature and relative humidity (RH) values are 58 F (WB 57.5) and 99%, respectively. In order to elevate the RH to 55% when plotting a psychrometric chart, the dB must increase to 68 °F (dt 10 °F). The heater will therefore be 400 * 10 * 3160 = 1.27 KW. Am I correct?

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[HVAC-Novice]

An indirect fired heater doesn't add or remove water. Look at psychrometric chart what the schange in sensible temperature does to RH.

 

[nuuvox000]

I'm seeing that you need to increase the temperature to 74°F to get to 55% from your starting point (at least at sea level).
I would recommend an electric heater with SCR control which can ramp the heat output up and down and dial in the temperature you want.
As far as the equation, it's qs=CFM*AF*1.1*dT
AF=altitude factor
dT=change in temperature
qs=sensible heat
CFM=cubic feet per minute
If you're at sea level, it would be 400*1*1.1*16= 7040 BTU/hr = 2.06 kW.
I would size it for 3 kW w/ SCR control.

 

[moideen]

@nuuvox000: thanks for the response, As per the psychrometric chart, 10F dt is enough to decrease the RH of 58F with 98%RH to 55% RH.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[MintJulep]

No.

Where did all that water come go?

The assumption that WB temp stays constant is not correct.

 

[BronYrAur]

Air at 58 deg F DB and 57.5 deg F WB is roughly 97%Rh as you indicated.

Heating this at 10 deg to 68 deg F DB, will result in roughly 68%Rh.

What %Rh are you trying to achieve, and why?

 

[moideen]

@MintJulep;Yes, you are right. The wet bulb is changing with sensible heating. So, I changed DB and RH.

@BronYrAur;I want to provide 55% to 60% RH fresh air

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[BronYrAur]

I'm away from my psych chart and computer. But I don't think it's possible to reach your desired condition without additional dehumidification. Why do you need 55% RH at 68°? Just curious

 

[moideen]

This duct heater follows the cooling coil. As we all know, the supply air will be nearly saturated after the dehumidification process by the fresh air system's cooling coil. Heat pipes in air handling unit do the same thing, so the goal is to keep the relative humidity between 55 and 60% by reheating and assisting in keeping the space RH at that level.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[MintJulep]

Putting 55%RH air into a space is generally not how to keep the space at 55%RH.

 

[ProcessHVAC]

The screenshots of your software show a drop in water content which means that to achieve your design point you need to reduce your water content, which you will not be able to do with a sensible heating process. I think need you to consult your application with a qualified mechanical engineer that understand psychrometrics.

 

[moideen]

@ProcessHVAC:I think you didn't read the whole post. After the cooling coil, the text says "sensible heating," which means "heating the de-humidified air after the cooling coil." The heater in the duct comes after the cooling coil. In the first screen shot, I apologies there is a mistake that is causing the DB to change with a constant wet bulb. Look at the second screen shot, please.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[BronYrAur]

@moideen You can't start at 58 deg F @ 98%rH and sensibly heat your way to 68 deg F @ 55% rH. You need additional dehumidification to achieve 55%Rh at 68 deg F dry bulb.

 

[ProcessHVAC]

@moideen. Sorry, but I am not sure you really understand psychrometrics and if this is a real building and not "millennial homework" assistance, you are about to make a mess. Plot the two points on a psychrometric chart, does 68 F @ 55% RH follows a sensitive heating process off 58 F sat?. Then...you need to dehumidify further. I am not even convinced you need 68 F @ 55% RH if you do you will require about 50 F sat to sensibly heat up to 68 F @ 55% RH.

 

[moideen]

@ProcessHVAC ,BronYrAur-I plotted the same values using a psychometric chart; you are correct, thank you for pointing out the error. 55% RH is not possible at 68°F (20°C).

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[AdrienneGC]

If I understand your system correctly, you are using the reheat coil as a means to allow the AC system to keep running to satisfy the latent load without overcooling the space. There is a lot of good information in this post - I just came to add this link to a Reheat/Heater Size Calculator which should be helpful for your calculations.

Adrienne Gould-Choquette, P.E.

http://www.adicotengineering.com

 

[trashcanman]

All the heating coils in the world WILL NOT REMOVE THE MOISTURE. You need to remove moisture by one of 2 methods:
1. Cooling coil
2. Dessicant coil

After reducing the moisture level, then reheat to design dry bulb temp., like hospitals used to do.

 

[moideen]

@trashcanman "All the heating coils in the world WILL NOT REMOVE THE MOISTURE"-First, go back and read the original post again; it clearly stated relative humidity, not absolute humidity. 

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-