[Equest-users] Baseline Chilled Water Loop

Tue Nov 16 07:23:21 PST 2010

As I indicated earlier, everyone has their own opinion about this credit.  But consider this:

The pressure drop across a cooling tower and condenser water piping system is roughly the same as the pressure drop thru the chiller evaporator and primary loop of a primary/secondary chilled water plant (on a per gpm basis once you correct for the higher flow rates of the condenser side).  In other words, on a PER GPM basis, a typical water-cooled chilled water plant will have similar pump HP per GPM for the condenser water system and primary chilled water system.  Not identical, but similar.  I’ve got 10+ recent base-building designs that confirm this.  90.1 requires 19 W / gpm for a condenser water system, and you are saying that you only model 6 W / gpm for the primary chilled water pump?  That is below realistic design values for a primary system, at least in my opinion.  

Example:  1,000 ton chiller at 54/42 requiring 2,000 GPM.  6 W / gpm thru the primary loop would only use 12.0 KW, or a 15 hp motor frame.  If you assume a chiller evaporator loss of 15 ft, and make only modest estimates of an additional 20 ft for piping losses, strainers, mixing valves, bypass valves, etc (35 ft total), you can try and select any pump you want but you’re going to need at least a 25 hp motor.  And with most chilled water plants, even 35 ft seems low for the pressure drop across a primary loop.  I don’t see how 6 W / gpm can accurately represent a chilled water primary loop even in the most efficient central plant designs.  

Furthermore, if ASHRAE had wanted us to prorate the 22 W / gpm between the primary and secondary loops, wouldn’t they have instructed us to do that, especially since there is a huge difference in energy consumption depending on which of the two loops has the higher W / gpm?  Have they instructed us to do that in an addendum?

I stand by my use of 22 W / gpm for each pump, and have never had a reviewer instruct me to do otherwise (which of course doesn’t mean that I’m not wrong).  

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

www.ghtltd.com <http://www.ghtltd.com/> 

From: Richard Ellison [mailto:REllison at southlandind.com] 
Sent: Tuesday, November 16, 2010 9:27 AM
To: James Hansen; John T. Forester; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Baseline Chilled Water Loop

I usually don’t respond to the information on this forum but I completely disagree with this latest e-mail.

The chilled water is supplied by a chiller not a pump.  “The baseline design pump power shall be 22 W / gpm” is for the GPM delivered by the chiller and the pumping system does not matter. The power is specified per GPM not per pump.   If you use primary/secondary then the energy is divided between the pumps.  We usually assume 6W/gpm for the primary pump and 16W/gpm for the secondary pump.  If one uses 22W/gpm for each pump, then the total of 44W/gpm the pump energy is huge and the savings unrealistic.

R I C H A R D  E L L I S O N
PE, CEM, CEA, CTAB, CBEP, LEED AP 

Energy Engineer 

Southland Industries

22340 Dresden Street, Suite 177

Dulles, VA 20166

Office: 703.834.5570

Direct: 703.834.2438

Fax: 703.834.5572

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Tuesday, November 16, 2010 8:50 AM
To: John T. Forester; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Chilled Water Loop

I’ve seen different people provide different responses to this question, but I’ve always modeled EACH pump in a chilled water system as 22 W / gpm.  Don’t prorate.

“The baseline design pump power shall be 22 W / gpm”.  It can’t be any more clear than this.  90.1 tells you when you need a primary / secondary system, and when you can get away with primary only.  In other words, they are telling you how many pumps you need to model.  And they specifically list the pump power in terms of GPM, which is PER PUMP.  Nowhere does it say to prorate.

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

www.ghtltd.com <http://www.ghtltd.com/> 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of John T. Forester
Sent: Tuesday, November 16, 2010 8:41 AM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Chilled Water Loop

Fellow Modelers,

I’ve received a comment regarding my response last week on how to model CHW pumps for App. G.  Specifically, item 7 (see below) and how the 22 W/gpm is defined for a primary secondary configuration.  Section G3.1.3.10  can be interpreted (at least) two different ways.

First, Section G3.1.3.10 Chilled-Water Pumps (Systems 7 and 8)

“The baseline design pump power shall be 22 W/gpm.  Chilled-water systems with a cooling capacity of 300 tons or more shall be modeled as primary/secondary systems with variable-speed drives on the secondary pumping loop.  Chilled-water pumps in systems serving less than 300 tons cooling capacity shall be modeled as primary/secondary systems with secondary pump riding the pump curve.”

The question: 

Should each primary and secondary pump be sized for 22 W/gpm OR should this “design pump power” be divided up (in some way) between the primary and secondary pumps?

Position A:

If the 22 W/gpm should be the TOTAL pumping power, there is no guidance on how to divvy up this requirement and the constant volume primary pumps could be sized for 20 W/gpm and the secondary (VSD or riding pump curve) can be allotted 2 W/gpm.  This would maximize the pump energy in the baseline model.

Position B:

Sizing each primary pump and secondary pump to 22 W/gpm will over-estimate the baseline pump energy (VSD or riding pump curve) and this is not the intent of 90.1.  The “design pump power” of 22 W/gpm is the same requirement as Section 11 (ECB method) just with a different pumping configuration.

How do you interpret (and model) 90.1 App. G chilled-water pump power requirement of 22 W/gpm?  (Your interpretation doesn’t necessarily have to be one of the positions above.)

Thanks,

John

John T. Forester, P.E., LEED AP, Mechanical Design Engineer I BVH Integrated Services I 617.658.9008 tel I 617.244.3753 fax I One Gateway Center Suite 506, Newton MA 02458 I www.bvhis.com I Hartford ● New Haven ● Boston

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of John T. Forester
Sent: Friday, November 12, 2010 5:47 PM
To: 'Smith, Michael'
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Chilled Water Loop

Hi Michael,

I think I can answer your App G question.

First, in the context of 90.1, “variable flow” does not necessarily mean variable speed.  It just means riding the pump curve with a bypass to minimize pumping pressure drops through the loop during part-load operation.

App. G Chilled Water Modeling <=300 tons

1.       Create a chilled water circulation loop and name it something like “Pri CHW Loop.”

2.       Create another chilled water circulation loop and name it something like “Sec CHW Loop.”

a.      Change the Loop Sub-type of this loop to Secondary and choose “Pri CHW Loop” as the primary loop.

3.       Create a circulation pump and name it something like “Sec CHW Pump.”

a.      Set this pump as variable speed.  I believe you are allowed to set the minimum speed to 50% for App. G. (someone here on the list can correct me on this if I’m wrong).

b.      By default, the valve on this loop is three-way and will circulate water directly back to the primary loop as long as there are two-way valves at your terminal units (AHU coils, FCUs, etc).

c.      Attach this pump to the Sec CHW Loop.

d.      Change the loop head sensor location to “at coils.”

4.       Create a condenser water loop and CW pump and attach the pump to the loop.

a.      Also create an open tower heat rejection device and attach it to your condenser water loop.

5.       Create 1 primary chilled water pump per chiller.  They won’t be attached to anything yet and will be at the bottom of the component tree.

a.      The default for a pump is “One-Speed.”  This is fine and the pump will act as a constant volume pump with the three-way valve on the secondary loop.

6.       Create a water-cooled chiller and choose the “Pri CHW Loop 1” and “water-cooled.”

a.      Since there is no pump attached to the primary loop, you will be required to select a pump for the chiller.

b.      You will also be required to select the Condenser water loop

7.       Now what you have to do is size your pumps so the total pump energy is 22 W/pm.  You will want to do something like 4-5 W/gpm for each primary pump and use the remaining for the secondary pump.

This system is primary/secondary with constant flow through each chiller and variable speed/variable flow through the secondary loop.  Since each chiller has a pump directly attached to it, each primary pump will automatically cycle on and off with the chillers.

John T. Forester, P.E., LEED AP, Mechanical Design Engineer I BVH Integrated Services I 617.658.9008 tel I 617.244.3753 fax I One Gateway Center Suite 506, Newton MA 02458 I www.bvhis.com <http://www.bvhis.com>  I Hartford ● New Haven ● Boston

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Smith, Michael
Sent: Friday, November 12, 2010 3:55 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Baseline Chilled Water Loop

Everyone,

I have a question regarding the baseline chilled water loop. 

ASHRAE 90.1 2007 Section G3.1.3.10 states that for large chilled water systems serving baseline systems 7 and 8 that are greater than 300 tons the loop “shall be modeled as primary/secondary with variable speed drives on the secondary pumping loop.”

The following section, G3.1.3.11 further goes on to say that “each chiller shall be modeled with separate…chilled water pumps interlocked to operate with the associated chiller.”

As clarification, does this mean three sets of pumps are required, or just two? Are the “primary” loop pumps specified as the chiller pumps? 

Two reasons why I am confused. Firstly, the Chapter 11 baseline chilled water loop is supposed to be simulated as primary-only variable flow, but still has the similar requirement for interlocked chiller pumps. That would be two sets of pumps. Adding a “secondary” loop actually adds a third set of pumps. 

The second reason why I am confused is that the Simulation Guidebook <http://www.energydesignresources.com/Portals/0/documents/DesignGuidelines/EDR_DesignGuidelines_%20HVAC_Simulation.pdf>  prepared for Pacific Gas and Electric (same affiliation as CoolTools) has a visual of how to simulate “primary/secondary” loops on page 52, but they do not include chiller pumps in their image of the eQUEST component tree. 

I can’t understand why three sets of pumps would be required. I supposed a follow up question may be if the 22 watts/gpm is inclusive of all three sets of pumps. Hopefully someone can clarify! I appreciate your help. 

Michael J. Smith, EIT, LEED® AP

WSP ▪ FLACK + KURTZ

512 7th Ave., New York, NY 10018

Direct: 212.951.2671| Main: 212.532.9600

www.wspfk.com <http://www.wspfk.com/> 

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