Uncommon heat load conditions, how to estimate coil capacity??

[SoD]

I am new to HVAC designing and I am working on an AHU as an intern.

I have some uncommon design conditions, for which I could not find any resources or help.

The cooling loads and ventilation requirements are very different than usual requirements. They are as follows,
Cooling loads: Sensible Heat load = approx 40%
Latent Heat load = approx 60%
Ventilation: 100% Outside air required, No return air

I haven't found any resources for calculating the capacity of coil.

If anyone could help me with the calculation method or resources that would be great. Thanks

 

[willard3]

Contact coil manufacturers for software that will do the calculation.

Talk to a senior Engineer in your office about coil sizing.

 

[BronYrAur]

Where did you get the 40% and 60%. Load will be determined by mass air flow and enthalpy differences. If you know your conditions, the coil can be selected rather easily.

You need to know:
airflow
entering air dry bulb and wet bulb
leaving air dry bulb and wet bulb

Your load, process requirements, etc determines the airflow and temp that you need.

 

[SoD]

@BronYrAur

The 40% SH and 60% LH load are just the occupants and equipment loads inside the room.
The outside air load would be different, that might just shift the load percentages but still the LH load is higher.

I am getting ERSHF = around 0.3
For which is does not intersect the saturation curve and hence I can not find the ADP.

Also I have no resources about the following doubts,
[ol 1]
[li]I don't have any experience to just select ADP according to any data.[/li]
[li]Also why reheat would be necessary for high LH load?[/li]
[li]What should be the bypass factor?[/li]
[li]And since it is all outside air requirement, do I have to calculate trials each time and select new ADP each time?[/li]
[/ol]

If you know any resources, or guidelines by your knowledge would be very helpful.
Also, I don't have any senior engineer over me to help.

 

[moideen]

SoD I thing you load related to 100% fresh air design. Treated/cooled fresh air will offset the internal heat gain. First you should calculated heat load by lights,pople,equipment..etc. find the required capacity and its required cfm. Then put in psychometric chart (mcquay psychrometric analyzer free in net)
Basic formula: 4.5*cfm*enthalpy difference= required capacity of coil.

 

[GT-EGR]

If you have your internal sensible and latent loads - from that you should have a target supply air CFM and it's associated dry bulb and wet bulb temperature that will satisfy both your sensible and latent loads. This is the airflow you need leaving your coil, regardless of how much outside air you are using.

Once you have that, the load on your coil is the BTU/h it takes to reduce that supply air CFM number, from your worst case outside air conditions (refer to ASHRAE tables in your location) down to your needed supply air condition. From there you do an enthalpy calculation to get the total coil load.

BTU/h = CFM x 4.5 x delta-enthalpy. The enthalpy values you have to lookup using your outside air dry bulb and wet bulb, and your supply air dry bulb and wet bulb (both were determined above).

 

[lilliput1]

Can you locate the exhaust pickup point close to the source of the latent and sensible heat loads to reduce the actual room load?

 

[SoD]

@lilliput1

The exhaust is to the roof. So it is far away from the heat source.
I have the following data from which I need to find the total capacity of AHU

1. Inside, Outside- DBT and RH
2. Ventilation rate
3. RSH and RLH
4. Room dimensions

Now the problem is when I calculate the sensible heat factor, it is very low and I am not getting an ADP for the given conditions.
So I have to assume the ADP which I don't know how to.

Also I have 100% fresh air requirement, hence it has to match the ventilation rate. I don't know the procedure for that.

So if anyone could provide any resources, pls help.

 

[lilliput1]

What are the equipment or objects in the room giving off sensible and latent loads? Give breakdown. What are the loads in Btu/hr? If latent loads are truly very high you may need to use desiccant type dehumidifier and regeneration system in addition to cooling coils. Can't exhaust be ducted to locate exhaust inlet close over the load sources?