[Equest-users] Chiller Curves (oh boy!)

Wed Nov 3 08:29:07 PDT 2010

Nick,

When defining a centrifugal chiller in eQuest, one of the items on the Basic Specifications tab under the Design vs. Rated Conditions is a Design/Max Cap ratio.  By default, this is 92% for a water cooled unit.  I believe this gets at the discussion in the help pages that talks about maximum capacity versus design capacity and how the chiller vendor will spec a piece of equipment.  Typically, vendors don’t often get asked (or provide) what the “Maximum” capacity of a spec’d unit is.  Therefore the performance data that they provide are at “design conditions.”

If you change the chiller type to a reciprocating chiller, this “Design/Max Cap” ratio is disabled and the default specified condition changes from “Design Conditions” to “Rated Conditions.”  This suggests that there is little “extra” capacity when a selection is done for that type of chiller.

If you do get “maximum capacity” data and create curves from that data, you will want to change the Design/Max Cap ratio to 1.0 so eQuest knows that there isn’t any spare capacity at the chiller.  Also if the data points you are using to normalize your curves are different than the design conditions for your energy model, you will want to change the “Chiller Specified at” value to “Rated Conditions” and enter the rated conditions for CHW temp, CW temp and CW gpm/ton to match your normalized curves.

Hope this helps,

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<http://www.bvhis.com> I Hartford ● New Haven ● Boston
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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Wednesday, November 03, 2010 11:03 AM
To: Carol Gardner; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Chiller Curves (oh boy!)

Thanks for the response Carol!

That 120% load case is what I’m getting at – let me try to explain a little further:

In the DOE2 help files, the vocabulary for centrifugal chillers is “design capacity” and “maximum capacity,” where “design” means the capacity at the rated or designed conditions (at which you define / specify your chiller), and “maximum” means the capacity the chiller is really capable of under the same conditions if it runs balls-out (maximum power to the refrigerant drive).

The help file excerpt I copied below with the red line is pretty explicitly telling us to normalize the part load values to the maximum capacity for centrifugal chillers.  I’ve highlighted a second line for clarity.  The EDR guidelines I linked below are saying you can instead normalize to the design capacity for the EIR-PLR curve if that’s all your field measurements or manufacturer rep can provide.

I’m asking – are both approaches right?

My first and second questions are kinda tied together… How would choosing to normalize to either the maximum or design conditions affect how we should handle the DESIGN-PLR ratio, if at all?

~Nick.

[cid:image001.jpg at 01CB7B4A.52750C30]

NICK CATON, E.I.T.
PROJECT ENGINEER
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
Check out our new web-site @ www.smithboucher.com

From: Carol Gardner [mailto:cmg750 at gmail.com]
Sent: Tuesday, November 02, 2010 10:04 PM
To: Nick Caton
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Chiller Curves (oh boy!)

Let me take a crack at this. If by design capacity you mean the chiller running at 100% load, you would create the curve(s) by normalizing around your ARI design conditions i.e. the PLR curve would be 1.0 at this point, call it ARI Cap and the other points would be 90% Cap/ARI Cap, 80% Cap/ARI Cap, etc. The same would go for your temp curves. If, however, your chiller is operating at 120%, or some such other level, I would normalize the curve around the ARI design conditions of the chiller at 120%. I had to do this for a VRV hp that was selected at the 120% design condition.

I find this from the DOE2 manual the most helpful:

Volume 2: Dictionary<volume2dictionary.htm> > HVAC Components<hvaccomponents.htm> > CURVE-FIT<curvefit.htm> > INPUT-TYPE = DATA<inputtypedata.htm>
INDEPENDENT-2

Used for all curves having two independent variables. A list of up to twenty values of the second independent variable. The number of values should be the same as for DEPENDENT.

Example 1: defining a curve by inputting a set of data points.

A packaged system (PZS) has cooling performance significantly different from that used in the default  model. The manufacturer lists the data shown in Table 46, for cooling capacity, at 2000 cfm design air flow rate, as a function of outside dry-bulb temperature and entering wet-bulb temperature.

Table 46  Cooling capacity (kBtu/hr) vs. temperature

Outside
Dry-bulb

Entering Wet-bulb

72F

67F

62F

85F

69

65

60

95F

68

63 (ARI)

57

105F

65

60

53

115F

62

55

49

In this example the independent variables are the entering wet-bulb temperature and the outside dry-bulb temperature. Because there are two independent variables and they have units of temperature, we input a curve of TYPE BI-QUADRATIC-T using the given data points. The dependent variable is not the cooling capacity listed in the table but rather the cooling capacity divided by the cooling capacity at the ARI rating point (95 F outside dry-bulb and 67 F entering wet-bulb). In other words, the capacities should be normalized to the ARI rating point., as shown in Table 47

Table 47  Normalized capacity vs. temperature

Outside
Dry-bulb

Entering Wet-bulb

72F

67F

62F

85F

1.095

1.032

0.952

95F

1.079

1.0 (ARI)

0.905

105F

1.032

0.952

0.841

115F

0.984

0.873

0.778

The CURVE-FIT input will look like the following:

CAP-CURVE-1 = CURVE-FIT
TYPE               = BI-QUADRATIC-T
INPUT-TYPE         = DATA
DEPENDENT          = (1.000,1.079,0.905,1.032,0.952,0.841,
                      0.984,0.873,0.778,1.095,1.032,0.952) ..
IN-TEMP1           = (   67,   72,   62,   72,   67,   62,
                         72,   67,   62,   72,   67,   62) ..
IN-TEMP2           = (   95,   95,   95,  105,  105,  105,
                        115,  115,  115,   85    85,   85) ..

Example 2: Defining a curve by inputting coefficients

We want a furnace to have a constant efficiency as a function of part load. To do this we must replace the default FURNACE-HIR-FPLR with a curve that will give a constant efficiency. The curve TYPE is QUADRATIC in the part load ratio (PLR). PLR correction curves are always multiplied by the unit capacity, not the load, to obtain the energy (fuel or electricity) use. Thus the curve we want is: 0.0 + 1.0*PLR + 0.0*PLR*PLR. The input will look like:

New-Furnace-HIR-fPLR = CURVE-FIT
TYPE               = QUADRATIC
INPUT-TYPE         = COEFFICIENTS
COEFFICIENTS       = (0.0,1.0,0.0) ..

Then in the SYSTEM command we include:

   FURNACE-HIR-FPLR = New-Furnace-HIR-fPLR

On Tue, Nov 2, 2010 at 3:21 PM, Nick Caton <ncaton at smithboucher.com<mailto:ncaton at smithboucher.com>> wrote:
Hi everyone!

I think I have finally wrapped my mind completely around custom chiller performance curves for a centrifugal VSD chiller.  I’ve got a few specific questions now that I’m on the other side of the fence:

1.       Is it necessary for the data points of a part load efficiency curve (EIR-FPLR&dT in my case) to originate from data with a 1.0 (100%) PLR ratio corresponding to a maximum vs. a design load capacity?  From what I gather in the EDR reference<http://www.energydesignresources.com/Portals/0/documents/DesignGuidelines/EDR_DesignGuidelines_%20HVAC_Simulation.pdf> (re: “Method 2” on PDF page 32/65), this curve can be generated using part-load readings assuming a design capacity at the 100% loading mark… but the DOE2 help entry for “EIR-FPLR” seems to suggest otherwise (copied below – see highlighted line).

2.       If the above part load efficiency curve is created based on data where the 100% loading point corresponds to the maximum (not design) capacity, should “DESIGN-PLR” (the ratio of design to maximum capacity) be set to 1.00 and the capacity of the chiller be specified at its maximum (not design) for the design/rated conditions?  As I write this question it sounds like I’m chasing my tail – someone straighten me out =)!

3.       When you veterans finish a project with sets of custom performance curves, do you have any suggestions for a naming scheme for future reference/re-use?  I’m currently thinking to keep the curves grouped in an .inp snippet I for importing along with an equipment cutsheet… but I’m certain I’ll forget the all the details as quickly as humanly possible when this project is behind me…

 ~Nick

[cid:image001.jpg at 01CB7B4A.52750C30]

NICK CATON, E.I.T.
PROJECT ENGINEER
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
Check out our new web-site @ www.smithboucher.com<http://www.smithboucher.com>
EIR-FPLR
Takes the U-name of a curve that adjusts the electric input ratio as a function of
• The part load ratio (PLR) –  The PLR is defined as the ratio of the hourly load to the hourly capacity;  Load / Caphour
• The evaporator/condenser dT -  The temperature differential between the condenser and leaving chilled-water. The  meaning of the condenser temperature varies according to condenser type.
For most chillers, the dT has a relatively small effect on part-load performance. However, for variable-speed centrifugal chillers, the effect of dT is as important as the PLR.  This is because the pressure rise across the impeller is proportional to the square of the impeller’s speed. Unless some form on condenser temperature relief is employed to reduce the temperature (and pressure) differential across the chiller at part load, the performance of a variable-speed chiller may not be significantly different than that of a constant-speed chiller.
To model power consumption as a function of the PLR only, use a CURVE-FIT of TYPE = QUADRATIC or CUBIC.  To model as a function of both PLR and dT, use a BI-QUADRATIC-RATIO&DT curve. The curve must be normalized to 1.0 at full load and the rated temperature differential.
Note that, for centrifugal chillers, ‘full load’ is defined as the ‘maximum capacity’, not the ‘design capacity’. Refer to the DESIGN-PLR keyword for more information.

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--
Carol Gardner PE
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