Trane Trace Design Airflow

[MoeEng]

Hello,

I'm a beginner in Trane Trace 700, and I'm having a problem I can't figure out. I'm doing load calculations for a building. I ran the calcs and everything came out OK. The procedure for me now is to go through the OA calculations using ASHARE 62.1 excel sheet (the company advises us not to use Trace ventilation method). Now that I hve the new OA numbers, I'd like to add them to the rooms, but I also want to specifiy the design airflow for the room so I don't have to go through a new set of OA calcs when Trace outputs a new design CFM.

I attempted to set the "Main Supply" airflow in the airflow tab for the rooms, but the airflow was still off by around 10%. I can't figure out why, but the design/space CFM is not coming out to what I specified. Does anyone know what the problem might be?

 

[EnergyProfessional]

I'm not sure if you mix up OA and design aiflow. Your company is right to require separate OA calculation.

You should call Trane support ( it is free with your nandatory subscription).

 

[Drazen]

do you swear that you have read manual in detail? if you don't do that, you will never reach acceptable level of knowledge and tips will not help you.

i don't use trace, but common fault in such setups is when concepts of supply air and outside air are mixed. most of load calcs software provides supply air calculation, and fresh air setup stands somewhere besides that.

 

[MoeEng]

By design airflow I mean the SA CFM, not the OA. I contact Trane support as HerrKaLeun suggested and they were able to help me out (not at once, but they got back to me). To who's interested, the answer is below.

To specifiy the desired design SA CFM, the block cooling airflow must not be filled out for the system. That's the only way to specify the SA CFM. Of course, that means that for a single system, you'd need to specify all of the zone SA CFM's so that you get to your block cooling airflow (from the AHU).

 

[cry22]

you may have locked the supply air temperature off the coil.
leave it blank and it will give you a different CFM
BUT, watch out if you get a too high SA temperature, you'll end with dehumidification problems.
suggest that you give your file a generic name and post a TAF file for others to see.
You know how to make a TAF file, don't you?
If not, close the file, go to file/archive project, click on the TRC file and an archived TAF file will be generated, post that file and we can check it out for you.

 

[MoeEng]

I do have the SA temperatures locked, and I'd like to keep them that way so I don't have to critically look at every zone in this building every time I adjust the Trace file. I do have another problem I'm trying to figure out, however.

For the outside air calculations, we design based on the peak design CFM for the VAV (in our case VAV with hot water RH). I got the OA numbers I needed to satisfy ASHRAE 62.1, and I inputted that into Trace as a CFM value. I set the SA CFM to the peak design of the VAV box.

However, I'm designing the system based on the Block CFM, not the peak. Regardless of which CFM I'm designing to, the CFM/ton comes out to around 220 which isn't easy to get fromn an off-the-shelf AHU. When I got selections, the CFM for the AHU was 65% over the design CFM specified by Trace, in order to meet the cooling load requirements.

What would be the procedure now? Should I bump up all of the VAV CFM's in order to get a total CFM that matches the AHU, re-do the OA calculations, and re-input the values in Trace? Or is there a standard method for adjusting the OA and load calculations based on the selected AHU? As Cry22 mentioned, the SA temp should change since the cooling load is relatively the same, but the CFM is a lot higher. However, the numbers in Trace stil have the specified SA temperatures, so I'm having trouble figuring this out.

Any help would be greatly appreciated.

 

[MoeEng]

I can archive it later today and post it if it'll help.

 

[EnergyProfessional]

Trane support also can take your archived file and review it if needed. That can take them a day or two, though.

 

[MoeEng]

Well the question I have now isn't necessarily a Trace question. It's what the standard procedure for matching your load CFM from your load calculations and your CFM from your selected AHU.

 

[Drazen]

i am quite impressed on how complicated your posts are.

can we go step by step, for instance if you would reveal how trace's supply air calculations are tied to your change of oa settings, we could possibly go forward...

 

[EnergyProfessional]

In my experience it is easier to calcaulte OA requirement and minimum flow rates separately in Excel (and you also need to meet local code, since that may differ from ASHRAE). and then just dump that amount of air into the AHU (in Trane system settings you just don't do all the fancy ASHRAE stuff).

This is easier to understand, less error-prone. Also easeir to show to AHJ what you did.
I once did a full ASHRAE calcaultion in Excel and Trace and never really had 100% the same numbers, they always were a bit different whne you include all the ASHRAE loopholes. And this didn't even work well with exhaust.

Here in WI code for ventialtion is different from IMC/ASHRAE. So what i do is calculate OA for code, OA for ASHRAE and pick the higher number. In addition we have often huge exhaust requirements, that supersede the OA requirements. In tha tcase the OA is overdesigned anyway. This is not easily doen in Trane, if possible at all.

Trane is really really good. but if you don't understand something 100% you easily do it wrong and get totally wrong numbers.

 

[MoeEng]

I'm not sure how to simplify what I'm saying

I have the required OA calcs from an Excel sheet. I inputted those values in Trace and I'm getting the cirrect OA outputs. Now, I took the block capcacity, the SA CFM, and got selections for an AHU. The CFM/ton based on the Trace file was around 220, which is too low to get a good match from an off-the-shelf AHU.

The selection I got from the sales rep had a 65% higher SA CFM than the one Trace said I should have. The question is, what now?


PS. HerrKaLeun, yea Trace is funny like that. We're actually supposed to do energy modeling with Trace in a later phase of the project. I've never done it before and wonder how accurate the results will be. I heard somewhere around +/- 20%!

 

[Drazen]

ok, from the new post it looks like there is no direct link between your oa inputs and trace load output though it looked so in first posts...

i have to guess and say that block load and ventilation load are summed up to reach reference data for your ahu?

on the other hand, i also have to guess that you have no any other choice but to take some packaged rooftop with fixed ton/cfm capacity.

first you should do is to check whether calculated minimum cfm can be reached by controls with your picked ahu at all. if not, you simply have to change ahu selection concept, give up locking supply temperature or similar. changing supply air temperature should reduce collision.

it is also interesting to see how much block load differs from peak loads sum? if you are limited with ahu choices, there could be, for instance, possibility to split into two completely separate systems where two separate ahu's could fit.

no tool i use is a magic tool. best of them have most of useful options, but we always have to be aware that each of them have their own limitations.

 

[EnergyProfessional]

MoEng: Trace is probably the most accurate energy simulation since it actually has all the HVAC options. But since it is more detailed, the user can make more mistakes.

If you use Trace for load anyway, it is natural to use it for energy simulation. But you should know what you are doing.

 

[MoeEng]

Drazen: You guessed right in that I have no choice but to take some packed rooftop with a fixed ton/cfm capacity. I'll try re-doing the calculations to get a better fit. The peak loads and block loads are pretty close; a difference of about 10-15% max. But that's a good way of validating zones. Gotta keep that one in mind! Thanks for the help.

HerrKaLeun: Oh yea, I can't wait til I have to re-do the energy model 10 times to figure out what I inputted wrong haha.

 

[EnergyProfessional]

MoeEng: there are so many AHU options (DOAS, where you add OA etc.). supply temp... and Tran can simulate almost everything. just for giggles change ducted to plenum return and you see a difference.
You also can insert the actual equipment unload and ambient relief curves etc.

Many energy simulations are not detailed with HVAC options. So the user can't do much wrong.

If you enter everything right, Trace will be accurate.

One issue with all energy simulations: they simulate the assumed occupancy and usage and also assume the system was properly commissioned and works as designed. In reality this isn't the case.

 

[MoeEng]

Yea that's true. I did read something that mentioned that some projects needed some sort of testing of the facility 1 year after building "open-up" to validate the energy model. I think the term was called calibrating.

 

[EnergyProfessional]

I think you mean the "validation and verification" requirement. Unless the energy simulation was wrong, this isn't to calibrate the energy simulation. this is more to verify if the system was installed and commissioned as designed.

the energy simulation, in my opinion, is to decide on what system is to be chosen. the owner can use upfront cost, maintainability, energy consumption etc. to make a decision. it isn't to predict exact consumption.
just for the fact that the energy simulation uses TMY3 or some other weather data (which are an average design year) will make it impossible to compare one year to the simulation.

 

[MoeEng]

Oh I see. As far as using energy modeling to make better design decisions, it's a privelage I don't see here very often. From what I've observed at many companies in the NY/NJ area, energy modeling is usually used to fulfil some sort of project requirement such as LEED, and it comes so far into the design phase that it makes it almost impossible to redesign in time for the deadline's.

I'm hoping one day we can get a project where the client cares more about efficiency and long-term returns than initial cost and deadlines.

 

[EnergyProfessional]

Is it possible that your 220 cfm/ton number is caused by including OA treatment? yes for recirculated air rule of thumb is 400 cfm / ton. But for OA this number is lower depending on your climate.
Just an idea...