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

[Carol Gardner]

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
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
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