Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Donate Today!

Do you enjoy these
technical forums?
Donate Today! Click Here

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Diversity factor in load [i]estimation[/i] (or calculation).Helpful Member!(2) 

SLBforever (Mechanical)
16 Aug 05 17:09
When we estimate (or calculate) the cooling loads of a series of spaces located in several facades, we obtain the peak load for a certain hour of the day.
However, it's posible that not all the spaces have a peak load at that hour of day.
In that case the peak load, may not be the sum of the peaks previously calculated.
As far as I understand,we need a dinamic model not a static model, otherwise we are oversizing the Chiller capacity.
What will be the correct aproach to this situation?
Is there a empirical diversity factor to apply to each facade, or do we need to performe a dinamic modulation of the load calculation?
What am I missing here?

Thanks,

SLB


MintJulep (Mechanical)
16 Aug 05 21:51
If you know the occupancy patterns for each space then you can work that into your calculation easily enought.  A quasi-dynamic model is likely sufficient, perhaps calculate the peak load each hour.

When I say "know", I mean know for all time.  Occupancy patterns tend to change over time (Original tennents move out, new ones move in).

Diversity factors are, by their nature, estimates.  
Helpful Member!  AEBuck (Mechanical)
16 Aug 05 22:12
Others will follow, but I'll start with two things.

Yes ... if you are designing air and refrigeration systems that can respond to diversity - envelope cooling in this case - then you should be using software that calculates the combined (block) load of all such spaces for each hour of the cooling season.  I have been using Carrier's HAP program for a long time with no complaints.  Within the last year I compared results from this program versus results from two other programs used by others that modeled identical spaces - one program from a similar manufacturer and one from a source without a vested interest in the results.  The difference was alarming.  Subject to further investigation when time permits, I will continue to rely upon a program that has served me well.

Down and dirty ... With weather and envelopes similar to those in the Pacific Northwest, shave 20-to-30% from the sum of peak space cooling loads, to account for envelope diversity, and keep your fingers crossed.

For some systems, like constant volume with terminal reheat, air handling and refrigeration equipment cannot see envelope diversity.

So, you aren't missing much except a load calculation program that's trusted in your area.
Helpful Member!  HVAC68 (Mechanical)
16 Aug 05 22:32
I have also used HAP extensively.  You can "schedule" various internal loads like lighting, occupancy, etc., in the program to get a far more accurate results than the instantaneous heat load which is calculated the traditional way of Q = U A Delta T.

Also, HAP calculates 8760 hours in a year - i.e. 365 days x 24 hours a day.  Am sure there are many other programs which could do equally well or better.

Try the following link which gives more details.

http://www.xpedio.carrier.com/idc/groups/public/documents/marketing/811-584.pdf

http://www.xpedio.carrier.com/idc/groups/public/documents/marketing/811-585.pdf

HVAC68

SLBforever (Mechanical)
17 Aug 05 8:29
Thanks for your tips.

I've read Carrier's papers, and tried Carrier's HAP 4.1 program in a fiends computer, however, we still didn't find out how to generate the 8760 hour report, were we are suposed to see the loads in a hour-by-hour basis, space by space for the entire year.
We tried the HELP file with no sucess.
The manual we have is very simple and it doesn't give any help in this subject either.
One other thing:the weather file for the city were I live (Lisbon, Portugal),seems to have temperatures much higher than the real ones, especially in the winter months.The manual as a warning about that, but I still don't understand.
When performing a load calculation wouln't that give a wrong number?
  From your experince with this program can you give me some insight?


SLB
briand2 (Mechanical)
17 Aug 05 8:32
Diversity is fine, especially with varying solar load on different aspects of the building, but what figures do you ask the commissioning engineer to balance to (on a variable flow chilled water system serving fan coil units)?
SLBforever (Mechanical)
17 Aug 05 10:28
I haven't performed a calculation yet, so don´t have the water flow figures in each branch, or rise.
You talk about variable flow, so I presume you are considering 2 way valves in each coil.
The instalation were I work as 40 fan coils each with a 3 way valve. The water flow is costant.
Don´t we need to balance the instalation is this case too?
We need to garantee that when a valve closes the others remain unafected.

SLB
AEBuck (Mechanical)
17 Aug 05 11:02
In the top menu bar, under the choice for "Reports", the last two choices for "Print/View Design Data" and "Print/View Simulation Data" give you options to review loads hour-by-hour.  I typically do not review this detailed data.  I print system and zone reports under "Print/View Design Data", which automatically selects the time of peak coil loads.  In you choose to model a plant, I believe there are similar choices, but haven't been there for a while.

In the "Systems" menu, tab for "Sizing Data", highlight "Zone Sizing", there are various choices in two drop-down menus on the right with which you should become familiar.

In the "Weather" menu, on the first tab you may change any of this information to suit local conditions and the program will generate new annual temperature profiles.  We have a local ASHRAE chapter that provided detailed data for many locations in our state - data not found in the ASHRAE book series.  Not sure where you can find published data for your local conditions.  I have used long-term weather data from local TV stations for other places.  The US military also offers weather data obtained from many of their bases worldwide.  Don't have the title on hand, NAVFAC something.  Bottom line ... if you trust the design parameters you input, I have found no reason to disbelieve the results.

You receive one attaboy (gender-free) for wanting to learn how software works before using the results.

The Commissioning Agent should ultimately rely upon trend log data to indirectly confirm proper water flow rates by reviewing leaving air and room temperatures and set points during appropriate seasons.  The TAB Agent has a few options for their work, which you are free to specify.  You could request measurements of all units during several times of the day.  They could simulate diversity by partially closing isolation or control valves in non-peak zones.  DDC is handy for this.  Of course, the TAB report must document such things.  I typically use automatic (spring-loaded type) flow control valves in systems with many small coils.  These valves require nothing more than a quick pressure drop measurement for balancing.  From one or two such measurements of all units at different times, one can usually surmise if sufficent pressure would be available at different load conditions.  But only time will tell.  NEEB or AABC standards must address this as well.  I'll check and relay anything interesting.
SLBforever (Mechanical)
17 Aug 05 11:34
AEBuck:
Thank tou very much for your help!
I am going to "digest" all the information you gave me.
I'll post further developments.

SLB
AEBuck (Mechanical)
17 Aug 05 13:32
About the weather source I mentioned ... it can be found through the following link http://www.hnd.usace.army.mil/techinfo/UFC/UFC%203-400-02.pdf.  This source quotes the same design values as ASHRAE, but includes much more information including median of extreme values of 36F winter and 99F summer dry-bulbs.

NEBB (the correct initials) describes procedures for balancing variable flow hydronic systems in Section VII - basically as I described, by simulating diversity manually in various parts of the system.  Thus, TAB work requires more than one or two passes of measurements.
HVAC68 (Mechanical)
18 Aug 05 2:41
AEBuck has replied your query in a very detailed manner.  Regarding the temperature conditions being higher in winter - The program also indicates the source from which the temperature data has been obtained.  If you think what's indicated there (despite being from a reliable source like ASHRAE) is far from reality, then you can change these data to what you think is right, but, better get these data from an authentic source like your local met. dept.  However, the output would indicate that the source for for weather data is "User defined" as against "ASHRAE.....".

HVAC68

SLBforever (Mechanical)
18 Aug 05 10:10
AEBuck:

I´ve downloaded the report on the link you gave me. It’s very good allright. However I couldn’t retrieved the meteo data I’m interest in (Lisbon, Portugal).
I followed the instructions on the page 62 of the report:

1. Access http://www.afccc.af.mil.
2. If you are using a computer within the military, select .MIL DOMAIN.
3. Select the appropriate response at any Security Alert prompts.
4. Select the Get Products button on the left side of the page.
5. Select the Engineering Weather Data (EWD) link in the center of the page.
The AFCCC PRODUCT LOCATOR page will display.

However the Engineering Weather Data (EWD) appears as simple text.

I wonder if you could help?

SLB
SLBforever (Mechanical)
18 Aug 05 10:30
Still about the weather hourly data profiles in HAP:

I still don’t understand why we need the hourly data profiles to calculate cooling loads.

In HAP help menu:

...[i] Design Temperatures Tab / Weather Form[i/]

About Design Temperature Data.  The Design Temperatures tab contains information about cooling design day temperature and humidity profiles for the currently selected city.  Based on the design parameters for the city you selected, HAP constructs 24-hour profiles of temperature and humidity for all 12 months.  Each profile represents warmer than normal conditions and typical humidity levels that coincide with these temperatures.  Temperature and humidity profiles form the basis for cooling design load calculations and system performance calculations.

Note:  Profiles for all 12 months represent warmer than normal conditions.  A common misunderstanding is the belief that profiles in winter months represent colder than normal conditions.  This is not the case.  All 12 profiles represent design cooling conditions.  The availability of cooling design profiles for all 12 months is mainly important in tropical sites where peak cooling loads can occur at any time of year.  In addition, cooling design data for winter months is sometimes needed for special applications outside of tropical regions.
The Design Temperatures Tab allows you to view and edit the cooling design temperature and humidity profiles.  The tab contains two tables:

•    The Monthly Max/Min table on the left-hand side of the tab summarizes the design temperature and humidity profiles by listing the maximum and minimum dry-bulb and wet-bulb temperatures in each profile.  This summary of data is useful for quickly reviewing the variation of design temperatures and humidities during the year.  It can also be used to shift the entire profile up or down, or to expand or compress its range.

In the Monthly Max/Min table each row contains data for a different month.  Columns contain the maximum and minimum dry-bulbs and wet-bulbs for each month.  To adjust the profiles, simply modify the desired maximum and minimum values.  The design profile will be updated immediately to span the range between the new maximum and minimum values, but will preserve the original shape of the profile.  Data for the updated profile can be reviewed in the Hourly Detail View table on the right-hand side of the tab.

•    The Hourly Detail View table on the right-hand side of the tab lists the complete cooling design dry-bulb and wet-bulb profiles for all 12 months.  This table allows the hourly profiles to be viewed and to be changed on an hour-by-hour basis.

In the Hourly Detail View table rows contain data for the 24 hours of the day.  Columns contain dry-bulb and wet-bulb profiles for each month.  To edit data use the scroll bars to move to a month and hour and then modify the desired values. [i/]

Why the HAP uses "higher than normal conditions"?

If I change the Hourly Detail View by hand to more “realistic” values for my local site, do the cooling load calculation become affected?

Or this only happens in "tropical" weather conditions?


Excuse me if I insist in this kind of question but i would like to understand the program before I start to work with it in a professional way, and there is no Senior engineeer to help me ...

SLB
HVAC68 (Mechanical)
18 Aug 05 22:23
Question : Why the HAP uses "higher than normal conditions"?

Answer : Not sure what you mean by normal conditions.  As I mentiond earlier, the conditions indicated in the program are from a reliable source like ASHRAE.  If you say that the values indicated by that source is "higher than normal", then you need a more authentic and approved source.

Question : If I change the Hourly Detail View by hand to more “realistic” values for my local site, do the cooling load calculation become affected?

Answer : Yes, it will get affected.

Question : Or this only happens in "tropical" weather conditions?

Answer : By logic, any changes made in any type of weather condition, should affect the calculation.

HVAC68

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close