Hi All,
I don't mean to ask questions back to back days here, but I ran into roadblock. I found an equation for the compressor EIR to be:
Cooing EIR=(1/EER-0.012167)/(1/3.413-0.012167)
Has anyone tried to derive this, or know of a document that has done so? I seem to be missing something in my derivation as follows:
* EER=Cooling Output (Btuh)/(Supply Fan Input (W) + Compressor Input (W))
* 1/EER=Supply Fan Input/Cooling Output + Compressor Input/Cooling Output
* Compressor Input/Cooling Output = 1/EER - Supply Fan Input/Cooling Output
At ARI condition supply fan is rated at 365W/CFM & 400CFM/Ton
So Supply Fan Input/Cooling Output = 0.146 kW/Ton or .012167 W/Btuh
* Compressor Input/Cooling Output = 1/EER - 0.012167
* Compressor Input/Cooling Output (Btu/Btu) = 3.413*(1/EER - 0.012167)
i.e.
EER=11
Compressor EIR = 3.413*(1/11 - 0.012167) = 0.2687 (should be 0.2580)
Is there something that I am missing?
Thank in advance,
Daniel Kaler
Dan,
Your sign is flipped on in the denominator. It should be (3.413-1 + 0.012167). I'm attaching a sheet that went around a while ago about converting from EER to EIR that goes into more detail. Hope that helps.
Brendan Hall, M.Sc., LEED AP BD+C
And on packaged air cooled equipment the cooling EIR represents the compressor and the condenser fan power.
Paul Riemer, PE, LEED AP BD+C
Another take on this is that we thought the packaged equipment included just the compressor and the condenser fan power, but not the indoor fan power (i.e. ventilation fan, supply fan, etc.). However, there's a bit more to this.
I've seen several submittals on packaged equipment where the published EER value does not include the indoor fan. So, we were originally thinking that eQuest should not be assuming that an entered EER value includes the indoor fan power either, for packaged equipment.
However, we have observed otherwise in eQuest. I can best explain via example.
Create a sample project in the wizard with a building big enough to require a PRTU > 760 MBH (63 tons). Per ASHRAE 90.1-2007 table 6.8.1A, the rated efficiency would be 9.5 EER.
Take the project to the detailed interface, however, and look at the EIR, which is 0.3051, which translates back to an EER of 11.19.
This means that eQuest used that same equation (that is used for the smaller systems) to also adjust the EIR for the larger systems to exclude the indoor fan power allowance
Therefore, eQuest thinks the EER we enter in the wizard includes indoor fan power, and then adjusts accordingly.
In researching this with the Trace program, Trace assumes in similar fashion ==> EER ratings for 90.1-2007 Baseline library members are assumed to include the indoor fan.
In reading AHRI 340-360, it appears that the AHRI 340-360 EER rating should include the indoor fan, but there's a catch
If it's a split system (i.e. indoor fan not part of unit), then a fan power allowance of 365 W/KCFM is to be used (i.e. see page 6). However, if the unit includes an indoor fan (i.e. packaged system), then the fan power is included in the EER rating, but the rating is based on a minimum external resistance per table 5 (page 7). Table 5 provides minimum external static pressure as a function of capacity ratings ==> ESP ranges from 0.20 to 0.75 as capacity varies from 65 kBtu/hr thru 505 kBtu/hr plus. Those ESP values seem a bit low as compared to ESP values I've seen for actual PRTU's. My opinion on that is that we have to be careful that a published EER value is using an unrealistic assumption for fan power as compared to what the unit is really designed to operate at.
We concluded that for the Baseline systems, you just have to be aware that eQuest is doing the same adjustment to strip out the indoor fan power, for all air cooled systems, split and packaged, for all sizes. We also concluded that the 90.1 baseline efficiency numbers include an allowance for indoor fan power, which appears to be consistent with AHRI 340-360.
However, it seems to me that eQuest should be using a different equation for the adjustment for larger systems (than it uses for the smaller systems) since the AHRI 340-360 table 5 indicates that the fan power allowance is a function of equipment capacity. eQuest's assumptions for fan power allowance do not appear to be consistent with the numbers currently used for the AHRI 340-360 EER ratings, for the larger systems (i.e. > 65 kBtu/hr). However, regarding the adjustment that eQuest is making, it may not be perfect, but it is much better than making no adjustment at all so we have to keep that in mind (i.e. more correct than not).
For the Proposed Design equipment, we just need to be careful to review the mfg submittals to make sure we have an EIR entered into eQuest that is based on just the compressor and condenser fans. Also, we should just calculate the EER ourself if we can, instead of using the published EER, since the published EER could be based on an unrealistic assumption for indoor fan power (which I believe it is, at least for large PRTU VAV systems). In this case, let eQuest do its thing from the wizard-to-detailed interface (i.e. stripping out indoor fan power allowance). However, be sure to go to the detailed interface and replace whatever eQuest came up for EIR, with the correct EIR value, which you have calculated based on just the compressor and condenser fan power.
This area was confusing for us at first, so I hope this will help somebody. Hopefully, I didn't make it more confusing.
If anybody has any follow-up comments, would love to hear them.
Thanks! : )
Regards,
JAH
James A. Hess, PE, CEM, BEMP