How Does Discriminator Control Influence Reheat Requirements in HVAC Systems?

[pinkcashmere]

Homework Statement

A factory has two zones. Zone 1 and 2 are both maintained at 75FDB. The design supply air flow rates to Zone 1 and 2 are 320 and 200 CFM, respectively. At a certain hour in summer, the cooling loads for Zone 1 and 2 are 6,000 Btu/hr and 3,000 Btu/hr, respectively.

Assume dry air conditions and air density of 0.075 lbm/ft3 .

(a) If a dual-duct constant-volume system is used with discriminator control for both cold and hot air, what would the required amount of reheat (Btu/hr) be provided by the hot air in each zone at the hour? The design cold and hot air supply temperatures are 55FDB and 100FDB, respectively.

(b) If a VAV system with 20% minimum position is used with discriminator control, what would the required amount of reheat (Btu/hr) be provided by a reheater in each zone at the hour? The design supply air temperature is 55FDB

Homework Equations

The Attempt at a Solution

I just want to know what discriminator control is in the context of this problem. I can't find any example problems like these on Google.

 

[russ_watters]

Welcome to PF!

I'm an HVAC engineer and this is the first I've heard the term. Googling, I found a Google books link to an old controls book that shows it as a selection algorithm for picking the control variable. In this case, that appears to be the maximum allowable supply air temperature to satisfy the warmer zone's cooling requirement without reheat...and at the same time, the lowest temperature to heat the cooler zone.

https://books.google.com/books?id=1...v=onepage&q=discriminator control vav&f=false

I say "appears" because this is an archaic control method (the dual duct part) that I'm not too familiar with.

 

[pinkcashmere]

Welcome to PF!

I'm an HVAC engineer and this is the first I've heard the term. Googling, I found a Google books link to an old controls book that shows it as a selection algorithm for picking the control variable. In this case, that appears to be the maximum allowable supply air temperature to satisfy the warmer zone's cooling requirement without reheat...and at the same time, the lowest temperature to heat the cooler zone.

https://books.google.com/books?id=1VZ-BgAAQBAJ&pg=PT180&lpg=PT180&dq=discriminator+control+vav&source=bl&ots=3r5Fj62Jld&sig=5nRiwlin8CAYWcOhspMU0k8sq7o&hl=en&sa=X&ved=0CBsQ6AEwAGoVChMI3bCxove_yAIVwxc-Ch3guAmV#v=onepage&q=discriminator control vav&f=false

I say "appears" because this is an archaic control method (the dual duct part) that I'm not too familiar with.

thanks very much for the reply!
I have another question

http://postimg.org/image/quz6smejb/full/
I don't really understand the basic conditions involving design/minimum volumetric flow rates to calculate reheat or to change the damper position. I was wondering if you could explain