[Bldg-sim] Cooling Design Calc Methods

[Jim Dirkes]

Dear  Ery,
Your point is well made!  Several things come to mind in response:

*         I was not aware previously and do not have enough data from this thread to be convinced that something like E+ is significantly better than the "originally a load calculation" software like HAP or Trace.

*         If I did have that conviction, I think it would be hard to convey that to the design engineer.  It's a hard sell to convince one of us stodgy engineers to use something different than our ASHRAE-compliant load software and take more risk (by sizing smaller) when we're in a big hurry and very design-budget-constrained.

o   On one project when I was barely out of the University, I sized a heating system here in Michigan USA with no "safety factor" because I was very confident of my calculations.  After its installation, the office area would not get above 20C / 68F when winter temperatures were near the design condition.  I checked every aspect of operation and triple-checked my calculations in order to fix the problem and save face - and failed.  My company didn't fire me when we had to pay for heating upgrades, but they were not happy!  After that, I resolved to "size generously with excellent part load operation".  I still think that is a reasonable approach, but I am smarter now and might still be willing to specify the smaller size instead of a larger size.

*         I do use E+ for load calculations when performing a detailed energy audit.  My conviction and my experience say that most equipment is oversized and if a piece of equipment needs replacing, we may as well recommend replacing with the proper size.  One recent project replaced the existing boiler with one 1/3 the original size!  (Another project kept the same size boiler.)

*         This brings up another major reason for oversizing.  If I need a 320 ton capacity chiller per my load calculations, I must choose between a 300 ton or a 350-400 ton machine.  Which will I choose?  Will internal loads from PCs go up or down?  If they go up, will my company be blamed for the cooling system inability to meet the load? Heck,  the 350 ton chiller doesn't cost that much more; I'm likely to play it safe!  It's worse on a percentage basis when my load is 5.5 tons and I must choose between a 5 ton and 7.5 ton system.

*         Finally, (can you tell I've thought about this before :)?) most of us Americans love to be comfortable ALL the time - at home, in the car, at work, while shopping and at the restaurant - no exceptions (We're all comfort wimps).  Even though the peak heating and cooling conditions only occur a few hours every year, few are tolerant of a little discomfort for those few hours.

*         Although all of the above is true, I know there are abuses in the design community that could be gently corrected.  As part of a recent commissioning project, I reviewed load calculations and found that the engineer was using 5W / sq.ft (>50W / sq.m) for internal loads ... in a standard office!  On top of that, he refused to make any changes when I pointed it out.  "Too much work to change and it will work OK as is."

*         All in all, the bottom line is probably something like "encourage excellence in the engineering community" and promote the benefits of excellence.   One of my mottos is "Excellence pays" - that is, excellence does not cost more, it pays you back.

p.s., In case readers are not aware of ASHRAE Standard 140, it is used to compare results of energy analysis programs for a set of standardized systems to a  reference calculation.  I don't think it focusses on peak loads, however.

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Ery Djunaedy
Sent: Friday, July 06, 2012 1:19 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Cooling Design Calc Methods

On 07/05/2012 02:21 PM, Jim Dirkes wrote:
Caveat emptor: I have not attempted this type of comparison, since my "day job" is energy modeling with E+, not load calculations.

Jim, shouldn't you do this on every project?

Our lab does not do design, but we end up doing this type of comparison on virtually every project. There is no point of energy savings if we are still designing an energy hog. Like you said, load calc is not our job. But its our job to educate the owner that their building must have an optimal load, and that has to be reflected in the system and plant sizes. ONLY when the load is optimized, then we can talk about saving some energy.

Of course there is ALWAYS some resistance (in various degrees) from the mechanical engineers, but we usually succeed in streamlining our inputs with the load calc inputs (read: to make their assumptions less conservative and reduce their safety factor, that we end up with an optimum peak loads).

I believe that you have a similar experience. Care to share with the forum about your experience on this?

Ery
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20120706/733aab9f/attachment-0002.htm>