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

Carol Gardner cmg750 at gmail.com
Wed Nov 3 08:32:39 PDT 2010


My gut is telling me that unless you are selecting your chiller to function
at 120%, or in effect making that the "design" condition, you should use the
100% capacity for your curve.

On Wed, Nov 3, 2010 at 8:08 AM, James Newman
<James.Newman at me-engineers.com>wrote:

>  Nick,
>
>
>
> I am also working away at the unsolved mystery of creating a chiller curve.
> I have come to the conclusion that if you are only inputting  an EIR-PLR
> curve, you must do this at ARI rated conditions because the other two
> curves, (CAP and EIR) are equest defaults at ARI conditions. I failed at
> making all three curves as my results ended up  cutting the energy use to
> about an 1/8. I think in order to make the (CAP and EIR) curves you must
> have a full range of chiller supply temps and condenser supply temps. The
> information I was provided was with the same supply and condenser temps with
> the variables being load and kw/ton. I’m going to try to construct a new
> curve with conforming everything to ARI conditions and see what the results
> come back as.
>
>
>
> *James M. Newman, EIT, LEED AP*
>
> Project Engineer / Energy Analyst
>
>
>
> *M-E Engineers, Inc.* <http://www.me-engineers.com/>**
>
> 10055 west 43rd avenue
>
> wheat ridge, co 80033
>
>
>
> office:  303.421.6655
>
> direct:  720.898.3148
>
> james.newman at me-engineers.com <catie.williams at me-engineers.com>
>
>
>
> *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 9:03 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.
>
>
>
> [image: cid:489575314 at 22072009-0ABB]**
>
> * *
>
> *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 <http://volume2dictionary.htm> > HVAC Components<http://hvaccomponents.htm>>
> CURVE-FIT <http://curvefit.htm> > INPUT-TYPE = DATA<http://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 curveof 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>
> 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
>
>
>
> [image: cid:489575314 at 22072009-0ABB]
>
> * *
>
> *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* *
>
> *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
>



-- 
Carol Gardner PE
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