Reheat single duct system

[ArsalanHabib]

Hi,

I usually use this forum when I don't understand anything and usually I find most of the answer through threads. I am trying to solve a problem here and I need some help. I am stuck with this problem I am sure any one in this forum can help me.

The problem is as follows.

Single Duct system with reheat serving two zones.

The thermostat setting is : 72degee F for heating & 78 degreeF for cooling.
Heating Load
Zone 1= 48000 btu/hr
Zone 2= 56000 btu/hr
Cooling Load
Zone 1=24000btuh
zone 2=50000btuh

For this system the supply air temp is 55degF all year round.

1.determine required zone air flow rates and capacity of AHU?
2. Calculate the reheat required for each zone and the temperature of the reheated air for each zone?

On a day at a certain time, the intermediate condition (off design) are given below
Heating Load
Zone 1= 24000 btu/hr
Cooling Load
zone 2=30000btuh

1. Without discriminator control, calculate the reheat required for each zone?
2. With discriminator control, calculate the reheat required for each zone and amount of saving?


I try yo solve it simply by Btus=1.09*CFM*deltaT but I am confused if I calculate the CFM through this using load BTUS and delta T by thermostat minus supply..... I don't understand am going to calculate amount of reheat and reheat temp. Please help I have a paper next week.

 

[willard3]

You should read forum policies, no 9 of which reads as follows:


>>>>>>>>Academic related posts are discouraged. If you post topics related to undergraduate course-work or projects, your posts will be red flagged and marked for deletion. Students generally have vast resources available to them (libraries, professors, etc), and need to learn how to use those sources instead of relying on the Eng-Tips.com fora (it is similar to having someone else do your homework for you; not a good idea.)>>>>>>>>>>>

 

[ArsalanHabib]

Hi,

Sorry I don't know about this rule. But I don't have any professor or any help and I left studies years ago and now I am attempting my credential recognition paper in Canada(I immigrated to Canada last December)and I really don't know any one here. I am just thinking you guys can help me.

 

[Drazen]

you can for example buy book of shane k. wang handbook of ac and r, mc graw hill, where you will find exact calculations for your problem, but if you do not have knowledge of thermodynamics and psychrometrics it will not help you.

it is not so simple that someone can help you by few tips.

 

[ArsalanHabib]

Thanks for the book reference. I have a pretty good knowledge of thermodynamics and psychometric, problem looks, very simple but I don't know why I am not getting right answers. Thanks.

 

[ProcessHVAC]

Try McQuinston's book. This is a simple problem if your fluid mechanics and thermodynamics are in good shape.

 

[ArsalanHabib]

I need opinion of you guys that I am solving that problem right or wrong.
I have the supply air temperature for all around the year and room thermostat temperature. So what I does first. I just assumed the RH 50% for room because that is what Reheat system for to maintain the humidity. Then through psychometric chart I locate the point through 50%RH and 72DegreeF and then heat is all sensible it means I go straight to 55 degree F in psychometric to extract the enthalpies at both point. Then I calculated the mass flow rate for both zones using the equation below.

q=m(Enthalpy at room condition – Enthalpy at supply condition)xDeltaTemp

For first zone I just put room load, Cp=.244 and room temp minus supply temp to get my mass flow rate for both zones.
Then I divide these flows with specific volume of supply air side to get my volume flow rates.
Now

q=mcpdeltaT

I have mass flow rate for each zone and after that I calculated Temperature of air leaving reheat coils below

48000=9600*.244*(Treheat-Troom)

Troom=72degreeF

Treheat after calculation Treheat=92DegreeF (That’s number make sense for me)
After that I calculated the Reheat required below

Q=9600*.244*(92-72)
Q=46848 BTU/hr

and I did the same for the second zone.

My other concern is about discriminator control. I think without discriminator control the system will provide the same BTU’s no matter what (But I am not sure). With discriminator control I assumed if load is reduced, it means outside temperature increases but I don’t know how to apply this in here. I am just thinking of calculating load by just putting q= 32000 instead of 48000, to calculate the reheat required and just subtract the design BTU’s to part load to get saving with discriminator control.

I appreciate your help Thanks.