[Equest-users] Air cooled chillers versus water cooled chillers

Mon Oct 19 06:57:36 PDT 2015

Rathna,
There are several inputs in eQUEST that impact your CHW plant pumping energy as modeled for Appendix G:

1.)    CHW pump power should be 22 W/gpm per G3.1.3.10. I don’t recall how “official” it is but many on this forum interpret this as for the entire primary/secondary pumping, meaning it should be split between the primary and secondary pumps. (For example, 10 W/gpm for the primary loop pump and 12 W/gpm for the secondary loop pump.) Splitting vs. not splitting is one of the major influences of Baseline pump power.

2.)    CW pump power should be 19 W/gpm per G3.1.3.11.

3.)    CHW coils should use CHW-VALVE-TYPE of TWO-WAY to facilitate variable flow through the secondary loop, regardless of using a variable-speed drive for the secondary loop pump (for cooling capacity ≥ 300 tons) or “riding the pump curve” with a single-speed pump (cooling capacity < 300 tons).

4.)    The loop HEAD-SETPT-CTRL and HEAD-SENSOR-LOCN have major influences on pump energy. There is no requirement in Appendix G. that specifies the type of control. In order of least to best efficiency, the settings are:

a.      FIXED; AT-PUMP

b.      FIXED; ENTERING-LOOP

c.      FIXED; AT-COILS

d.      VALVE-RESET; n/a

I think FIXED, AT-COILS is a reasonable setting for both the Baseline and Proposed models, unless the Proposed design has the controls to survey the positions of all coils and implement the VALVE-RESET strategy, and then I use that in the Proposed.

5.)    LOOP-OPERATION impacts all CHW end-uses (cooling, pumping, heat rejection) especially if cooling is not needed year-round. This is also not specified in Appendix G. STANDBY keeps the CHW plant running when any systems with CHW capability are running. DEMAND-ONLY turns the CHW plant off unless there is a coil or process load for that hour. I think either setting is fair as long as it is kept identical between the Baseline and Proposed models.
Besides pumping energy, there is an easily-overlooked Baseline requirement per G3.1.3.11 of axial fan cooling towers with two-speed fans. Table 6.8.1G shows the minimum performance requirement of ≥38.2 gpm/hp. You have to calculate the fan kW from this based on the autosized tower flow and enter it as FAN-KW/FLOW or EIR. The eQUEST default EIR of 0.0105 for open towers is actually more efficient than the Table 6.8.1G requirement.

Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Rathna Shree
Sent: Monday, October 19, 2015 1:03 AM
To: Sambhav Tiwari <tiwari.sambhav at gmail.com>
Cc: Equest Users Mailing List <equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] Air cooled chillers versus water cooled chillers

Hi Sambhav,

Does that mean, air cooled chillers with variable primary pumping system performance is almost equivalent or better than the ASHRAE 90.1 2007 defined water cooled centrifugal chiller system?

Regards,
Rathnashree

On Saturday, 17 October 2015 12:57 PM, Sambhav Tiwari <tiwari.sambhav at gmail.com<mailto:tiwari.sambhav at gmail.com>> wrote:

Hi Rathanashree,

For me the results seems to be alright

1 Being a air cooled chiller with less COP than baseline water cooled your compressor energy consumption is increasing which is clear from the results .

2 Now there is a debate going these day to replace ( constant primary & variable secondary pumps) with variable primary pump system although many HVAC experts  advocate against it that it can have negative impacts on chiller too.But definitely it will save energy consumption under pumps which is clear from your results your proposed case is left only with VFD based primary pump compared with baseline of ( constant primary+variable secondary+constant condenser water pumps) therefore pumps energy is too less and justified to me.

3 Your cooling tower energy consumption will be nil in proposed case being a air cooled system that is also fine.

But the main driver or  you can say main ECM in your analysis is variable primary and no secondary chiller which is contributing siginifant savings in pumps along with no cooling tower present hence your over all energy saving are positive.

Hope this may help.

Warm Regards
Sambhav

On Fri, Oct 16, 2015 at 11:50 PM, Rathna Shree <rathnashreep at yahoo.in<mailto:rathnashreep at yahoo.in>> wrote:
Dear All,

A project has 14 floors and 3.5 lakhs sq.ft. of conditioned area. The project is going for LEED certification. The baseline HVAC system will be Variable air volume system with water cooled centrifugal chiller of COP 6.1. LPD is maintained at 0.7W/sft. Building envelope is more or less equal to baseline case. In this scenario, the proposed HVAC system is air cooled screw chillers with variable primary pumping system. Even though the project is getting negative results in cooling category, there is a huge savings from pumps which is compensating for the negative in cooling energy consumption. Ultimately, the overall plant energy consumption (Cooling, heat rejection, pumps) is lesser in proposed case than base case. I am surprised with the result as i believe for such a building, water cooled chillers work more efficiently. For more clarity, the results are given as below:

Base case: Cooling: 14,26,659 KWH, Heat rejection: 1,23,357 KWH, Pumps: 426,506 KWH
Proposed case: Cooling: 17,89,814 KWH, Heat rejection: 0 KWH, Pumps: 62,264 KWH

These are results are for default chiller curves in both the cases. If air cooled part load values are used, then cooling energy consumption further reduces in proposed case. Then can it be concluded that air cooled with primary variable pumping system is a good alternative to use.

Is this conclusion appropriate? Are the results correct? Please help.

Regards,
Rathnashree

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