[Bldg-sim] FW:

[Andrew Craig]

An example of sizing Fan Terminal Units (FTU)....

 

I have a 1000 cfm zone (determined from peak load conditions) to be
served by a FTU, therefore the parallel fan in my box is sized for 500
cfm (50% of peak flow).  Now typically in heating is when the primary
air is going to go to an absolute minimum.  This must be 300 cfm or the
total volume of OSA that I need for the zone, whichever is larger.  In
theory, the fan could provide all 500 cfm of its air when the primary
air is at a minimum if that meets the load.  The fan will typically
always be the first stage of heating.  After that, if setpoint isn't
met, the reheat coil comes on.  I would say that the design condition,
depending on climate of course, will be cooling dominated 7 times out of
10.

Also, as a note, the discussion of minimum flow setpoint is an area of
weakness for a VAV system.  If the zone was designed for 1000 cfm in
cooling mode and the minimum outside air was determined to be 300 cfm,
you will almost never reach this value when the VAV is operating at part
load.  There is a good article on this in the January issue of
Engineered Systems magazine.

 

Hope this helps.

 

Regards,

 

Andrew Craig, EIT,LEED AP/ Mechanical Designer 
INTERFACE ENGINEERING 
direct 503.382.2696 

________________________________

From: Chien Si Harriman [mailto:chien.harriman at iesve.com] 
Sent: Thursday, February 21, 2008 2:25 PM
To: Andrew Craig
Cc: bldg-sim at lists.onebuilding.org
Subject: FW: [Bldg-sim] FW:

 

Hi Andrew, that is really helpful.  I missed that section of the code.

 

Now, a few points that maybe someone could elucidate, maybe even you:

 

1.)  Does 50% of the peak design flow rate imply that, if, for example,
the min vol setpoint is 30%, then the recirc coming in parallel is 20%?

2.)  That makes sense, except I am guessing this CFM value is the 50%
peak design flow rate?  That would only be logical.

3.)  This min setpoint is for the primary air.

4.)  The design condition they are referring to is the summer design
condition, or the winter design condition or both?  I am guessing it is
both.

 

 

That is a great help, thanks for everything so far.

 

Sincerely,

Chien

 

 

 

 

________________________________

From: Andrew Craig [mailto:andrew_c at ieice.com]
Sent: Thu 21/02/2008 22:12
To: Chien Si Harriman; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] FW:

Chien---

 

Section G3.1.3.14 of Appendix G hit on requirements for fan powered
boxes.  They are:

 

1.)     FTUs shall be sized for 50% of the peak design flow rate 

2.)     FTUs shall be modeled with 0.35 W/cfm fan power.

3.)     Minimum volume setpoints for FTUs shall be equal to 30% of the
peak design flow rate or the rate required to meet minimum OSA
ventilation, whichever is larger.

4.)     The supply air temperature setpoint shall be constant at the
design condition.

 

Hope that helps.  Good luck.


Regards,

 

Andrew Craig, EIT,LEED AP/ Mechanical Designer 
INTERFACE ENGINEERING 
direct 503.382.2696 

________________________________

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chien Si
Harriman
Sent: Thursday, February 21, 2008 2:07 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] FW:

 

 

 

________________________________

From: Chien Si Harriman
Sent: Thu 21/02/2008 21:46
To: bldg-sim at lists.onebuilding.org
Subject: 

Hi there-

 

I've been tasked with modeling a parallel fan powered box as my baseline
system for ASHRAE 90.1 Appendix G, and basically I am wondering if there
is anyone who knows of modeling guidelines for that FPB?  I have the
capability of modeling any sort of conditions I would like, so I can
vary the flow rate of the recirc air, the control sequencing of the
fan-assist, and even whether that fan is variable volume or constant
volume.

 

I can't find any guidelines that exist anywhere on this topic in ASHRAE
90.1, and this is the first time I've had to model it (I've never
designed an electrically-heated building before).  Does anyone know
whether:

 

1.)  There is a specific ratio for the amount of recirc air to minimum
primary air?

2.)  Whether the amount of recirc air should vary (ie-a variable speed
fan)?

3.)  You can just use a real FPB with data from a company like Carrier,
and just call it a day on the simultion effort?

 

 

 

Any guidance would be really helpful.

 

 

Thanks in advance,

Chien Harriman

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