[Equest-users] Chiller Curves (oh boy!)
Nick Caton
ncaton at smithboucher.com
Wed Nov 3 08:03:06 PDT 2010
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.
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 > HVAC Components > CURVE-FIT > INPUT-TYPE = DATA
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>
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/DesignGuidelin
es/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
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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