[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [EnergyPlus_Support] Purchase air question

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Purchase air question
From: "Michael J. Witte" <mjwitte@xxxxxxxx>
Date: Thu, 20 May 2004 15:01:10 -0500
Delivered-to: lawrie.com%linda@xxxxxxxxxx
Delivered-to: mailing list EnergyPlus_Support@xxxxxxxxxxxxxxx
In-reply-to: <Pine.LNX.4.58.0405181807530.11291@kronos.eng.auth.gr>
List-unsubscribe: <mailto:EnergyPlus_Support-unsubscribe@yahoogroups.com>
Mailing-list: list EnergyPlus_Support@xxxxxxxxxxxxxxx; contact EnergyPlus_Support-owner@xxxxxxxxxxxxxxx
Organization: GARD Analytics
Priority: normal
References: <20040518085945.92662.qmail@web86408.mail.ukl.yahoo.com>
Reply-to: EnergyPlus_Support@xxxxxxxxxxxxxxx

Purchased air works as follows:

1.  Determine required sensible load to meet the thermostat setpoint.

2.  Using the difference between the current zone air temperature and the 
specified purchased air supply temperature (for cooling or heating as 
appropriate) calculate the required supply air flow rate to meet the load.

3.  If capacities limits have been specified, then limit the air flow if needed 
and fail to meet setpoint.

4.  Once the airflow rate has been established, use the specified supply air 
humidity ratio at this flowrate to determine the resulting humidity ratio in 
the zone.  

5a.  If outside air is specified as infiltration, then it adds to the zone load 
and the purchased air system meets this total load.

5b.  If outside air is specified as part of purchased air, the purchased air 
model volume flow rate will be calculated as in step 1 above.  If the supply 
air flow rate is greater than the specified OA flow rate, the difference is 
assumed to be return air and the load will be calculated assuming this mixed 
air flow is conditioned to the specified supply conditions (temperature and 
humidity).  If the supply flow rate is less than the specified OA flow rate,  
the load will be calculated assuming 100% outside air at the supply flow rate 
being conditioned to the specified supply conditions (temperature and 
humidity).  This enthalpy difference is reported as the load.  This method will 
almost always be valid for determining peak loads.  If you are using purchased 
air to calculate annual space loads including outside air loads, this method 
may not be accurate.  Instead you should use method 5a.

Now to answer your questions from below:

a)  Does the specified purchased air supply temperature matter?  Normally, the 
answer is no, but there are situations where it could have a significant 
impact.  Changing the supply air temperature will alter the calculated supply 
air flow rate to meet the same load.  If the specified supply humidity ratio 
remains the same, different supply air flow rates may result in a different 
moisture balance in the space and different latent loads for purchased air.  
Also, if outside air is specified as part of purchased air, the change in 
supply air temperature could alter how often a low supply air flow rate occurs 
causing changes in the total OA introduced.

b)  Because the supply air in purchased air is forced to always be at the 
specified supply humidity ratio, the zone RH calculations will only be 
approximate.  Very few system types provide a fixed humidity ratio of supply 
air.  If zone RH is important to you, you really should model a full HVAC 
system.

c)  The normal solution model (without MTF or EMPD) does a full moisture 
balance on the zone air assuming no moisture storage or transmission in the 
building envelope.  The EMPD model adds moisture storage in the building 
surfaces.  The MTF model adds moisture storage and transmission in the building 
envelope.  Note that the current MTF model has very limited applicability due 
to the nonlinear nature of material moisture transport properties.  It is not 
suitable for an annual simulation.

d)  When OA is specified as part of purchased air, all of the OA flow is 
conditioned to the specified supply air humidity ratio.  This can result in a 
signficantly different humidity balance compared to introducing the OA as a 
zone load through infiltration.  So, if you are using purchased air to 
determine annual space loads including latent loads, the best approximation is 
to introduce OA as infiltration.

Mike

On 18 May 2004, at 18:28, Socratis Pandazis wrote:

> 
> Hello . I'm trying to simulate a rather large building in the Aristotle
> University In Thessaloniki, Greece. Right now i'm interested in
> determining the thermal and cooling loads, so i use purchased:air.I'd like
> to now how exactly the temperature of the supplying heating/coolin air
> affects the loads, e.g if the heating temperature is 25 celsius degrees
> and not 50 as in the example idf, what effect that would have on the
> thermal load of the zone? Or is it that actually there is no importance of
> the supplying temperature, and the results do not vary muchand why?
> 
> One last thing.I also want to calculate the relative humidity in the
> zone.I've seen in the rdd file that the appropriate variable exists,
> although i haven't used MTF as the desired solution algo.Is the MTF used
> only for determining the moisture transfer through the walls?Currently i
> use the purchase air component with no outside air ste, but rather use the
> infiltration method to handle it.
> With a comperative simulation i found thah when using purchased air with
> the outside air option, my rel.humidity was up to 97% and the humidity
> ratio was constant and the value of humidity ratio set in the
> purchased:air, while when using infiltration the re.humidity was down to
> 55-70% for the given dayand the reatio varied as expected.Which is the
> best way to simulate, given the fact that i wan also to see  the depedence
> of temperature and moisture with eachother in the loads simulation?
> 
> Thx for yout ime and i hope i am clear in my questions
> 
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 

========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx

------------------------ Yahoo! Groups Sponsor ---------------------~-->
Yahoo! Domains - Claim yours for only $14.70
http://us.click.yahoo.com/Z1wmxD/DREIAA/yQLSAA/dkFolB/TM
---------------------------------------------------------------------~->

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.

Yahoo! Groups Links

<*> To visit your group on the web, go to:
     http://groups.yahoo.com/group/EnergyPlus_Support/

<*> To unsubscribe from this group, send an email to:
     EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
     http://docs.yahoo.com/info/terms/

Follow-Ups:
- [EnergyPlus_Support] Loads breakdown
  - From: carmen herrero prada

References:
- Re: [EnergyPlus_Support] Crash Suddenly
  - From: ALI AL-AJMI
- [EnergyPlus_Support] Purchase air question
  - From: Socratis Pandazis

Prev by Date: Re: [EnergyPlus_Support] purchased air with other system?
Next by Date: Re: [EnergyPlus_Support] Polygon Problem.
Previous by thread: [EnergyPlus_Support] Purchase air question
Next by thread: [EnergyPlus_Support] Loads breakdown
Index(es):
- Date
- Thread