Replace HVAC Units (Indoor Package Ventilation Units) 3

[Gtmohvac]

Hello again everyone!

I'm here again to get some tips.

An existing package ventilation unit will be replace/retrofitted by a new one.

The present system works on with a bypass air return to the unit.

Th old unit supplies 8000CFM ventilation air passing through a reheat coil unit to an air condition of 75ºF DB/50%RH. A 3000CFM will be supplied to the room while the remaining 5000CFM will return to unit as bypass-air.

The outdoor air condition is 93ºF DB/ 82ºF WB and the existing unit total cooling capacity is 313,600 BTU/hr.

Anyone can help me to find the entering air condition and leaving air condition on the unit with the provided information?

 

[cdxx139]

Gt,

1. Your TBD point is a mixed air calc of the Outdoor Air and Bypass Air.
T(tbd) = (cfmOA x tOA)+(cfmBA x tOA)/cfmTOTAL

2. Your TBD downstream of fan is determined by unit manufacturer, but can approximated with BTU=1.08 cfm deltaT formula.

3. Your using condenser water to reheat? Never seen that before. The reheat coil should be sized for what the water conditions are available onsite. (90 entering and 80 leaving for typical cooling tower loops)

Good luck

 

[trashcanman]

This is a mixing problem. Your above data does not match the diagram. It says 78/50% to the room, not 75/50%.

So, you are mixing 5000 cfm @ 78/65 + 3000 cfm @ 93/82 (assumed at sea level)

Mixing results: 8000 cfm @ 83.47 db/71.79 wb, h1 = 39.65 BTU/# DRY AIR.

leaving coil: 78/65, h2 = 29.9 BTU/# DRY AIR.

TOTAL LOAD = 8000* 4.5* (h1-h2)
= 8000*4.5*(39.65-29.9) = 206784 BTUH

Thus, the coil has the capacity.

 

[Gtmohvac]

thanks cdxx and trashcanman!

Sorry for the data trashcanman.

the design room condition was 78/50%.

i did get the mixing air condition.

however, i'm still confused for the data you've given me for the leaving coil condition of the unit.

i'm sorry again for the lacking information. the existing reheat coil is not part of the unit.

can you verify again? thanks

 

[trashcanman]

You provided the leaving condition, 78 deg.db, 50% RH. That is approx 78 db/ 65 wb.

 

[Gtmohvac]

Thanks trashcanman. I appreciate it.

 

[Gtmohvac]

Hello Guys!

I'm back on this thread.

I have a few concerns regarding this subject. I hope i could get some help ffrom you guys out there.

Existing unit schedule stated that it supplies 9600 CFM and the replacement unit supplies 12000 CFM. The Bypass air will increase by as much as 20%, hence, the bypass air duct shall remain.

1. Is the bypass air duct capable of the new CFM requirement? (considering 22"x30" with 8500 CFM return/bypass)

2. If so, what are the items that need to be considered?

3. If not, I guess duct work should be modified...

 

[CountOlaf]

I find it interesting you have the word HVAC as part of your username--have u designed any of these systems before ?

First, if your not changing the fuction of the space and you want less airflow, you might look into adjusting/changing out the pulleys/belts to get the airflow down to where it was formerly.

Secondly, get yourself a ductulator. 8,500 CFM through a 22" x 30" duct will have a velocity of 2,000 FPM and a pressure drop of 0.2"/100' of duct. 2,000 FPM velocity is OK for offices and industrial bldgs (in terms of noise) but wouldn't be for a theater or library for instance. The additional pressure drop in the bypass section may not be an issue since it's probably not the "longest run" or critical path. You'll have to look at the external static pressure of the new fan and see if it will be adequate to deliver the desired CFM to your conditioned space given existing duct sizes.

 

[willard3]

Methinks gtmohvac needs an experienced Engineer to check his work.

gtmohvac: If your boss has given you this project and you don't understand it, tell your boss you don't know how to do it and need help. Don't try to fake this on a web board.

 

[Gtmohvac]

Thanks Count I appreciate it... this is my first time to handle a project like this...

And by the way, thank you too willard3... That's what I intend to do on this site... seeking healthy tips... and some motivational advise...

 

[HVAC68]

Why do you need the replacement unit to supply 12000 cfm, when you want only 9600 cfm ? Check with the manufacturer and see if you can change the pulley on the evaporator and get a lower air quantity ??? Does your cooling requirement get satisfied with 9600 cfm ?

HVAC68