Chiller curve ball

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Ok - so I'm probably doing this all wrong.

I am trying to model a York scroll chiller in EQUEST. The table below illustrates the Part Load rating Data

Load %

Capacity (Tons)

Cond. EWT (?F)

Cond. LWT (?F)

Compressor kW

EER

COP

EIR

100

135.8

85

95

104.1

15.6

4.6

0.217391

83.3

116.8

79.4

87.9

81

17.3

5.1

0.196078

66.7

96.9

73.5

80.4

60.1

19.3

5.7

0.175439

50

73.5

66.7

71.8

41.7

21.2

6.2

0.16129

33.3

48.2

65

68.4

26.6

21.7

6.4

0.15625

16.7

23.6

65

66.6

13.1

21.7

6.4

0.15625

eQUEST doesn't seem to have a scroll chiller option - I assume screw is the closest thing to it.

As far as I can tell - you need to define 3 performance curves:

1. A curve that defines the EIR based on the condenser water temp
"York - Temp vs EIR" = CURVE-FIT
TYPE = BI-QUADRATIC-T
INPUT-TYPE = DATA
INDEPENDENT-1 = ( 85, 79.4, 73.5, 66.7, 65, 65 )
INDEPENDENT-2 = ( 95, 87.9, 80.4, 71.8, 68.4, 66 )
DEPENDENT = ( 0.217391, 0.196, 0.175, 0.161, 0.156, 0.156 )
..

2. A curve that defines the part load based on the condenser water temp
"York Temp vs Part Load" = CURVE-FIT
TYPE = BI-QUADRATIC-T
INPUT-TYPE = DATA
INDEPENDENT-1 = ( 85, 79.4, 73.5, 66.7, 65, 65 )
INDEPENDENT-2 = ( 95, 87.9, 80.4, 71.8, 68.4, 66 )
DEPENDENT = ( 1, 0.833, 0.667, 0.5, 0.333, 0.167 )
..

3. A curve that defines the EIR based on the part load.
"York Part Load vs EIR" = CURVE-FIT
TYPE = QUADRATIC
INPUT-TYPE = DATA
INDEPENDENT = ( 1, 0.833, 0.667, 0.5, 0.333, 0.167 )
DEPENDENT = ( 0.217, 0.196, 0.175, 0.161, 0.156, 0.156 )
..

When I apply these curves to my chiller, my cooling energy goes up by 30%. Seems like a lot. Is this reasonable or am I doing something wrong with my curve selection?

Muchas Gracias in advance
Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Offline
Joined: 2011-09-30
Reputation: -1

You?ll need to get more data than this for the custom curve to work well ? I
think you can find some fairly recent discussion on one of the lists, but
the sample data from York only shows one load point at each ECWT, therefore
the eQuest curve won?t be accurate anytime that the load and ECWT don?t
match your data.

You?d prefer to have load and ECWT data in a matrix around all of these
points ? i.e. several load and kW data points at 85F ECWT, 75F ECWT, 65F
ECWT.

My first suggestions is to try using report variables to show kW and chiller
production while using the default curves. See if when conditions specified
below occur there is a reasonable match to the York data kW/ton.

If yes, use the defaults. If no, ask York for more data.

*
*

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP

*
*

*From:* equest-users-bounces at lists.onebuilding.org [mailto:
equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Sami, Vikram
*Sent:* Wednesday, March 09, 2011 5:54 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] Chiller curve ball

Ok ? so I?m probably doing this all wrong.

I am trying to model a York scroll chiller in EQUEST. The table below
illustrates the Part Load rating Data

*Load % ** *

* **Capacity (Tons) *

* **Cond. EWT (?F) *

* **Cond. LWT (?F) *

* **Compressor kW *

*EER*

*COP*

*EIR*

100

135.8

85

95

104.1

15.6

4.6

0.217391

83.3

116.8

79.4

87.9

81

17.3

5.1

0.196078

66.7

96.9

73.5

80.4

60.1

19.3

5.7

0.175439

50

73.5

66.7

71.8

41.7

21.2

6.2

0.16129

33.3

48.2

65

68.4

26.6

21.7

6.4

0.15625

16.7

23.6

65

66.6

13.1

21.7

6.4

0.15625

eQUEST doesn?t seem to have a scroll chiller option ? I assume screw is the
closest thing to it.

As far as I can tell ? you need to define 3 performance curves:

1. A curve that defines the EIR based on the condenser water temp

2. A curve that defines the part load based on the condenser water
temp

3. A curve that defines the EIR based on the part load.

When I apply these curves to my chiller, my cooling energy goes up by 30%.
Seems like a lot. Is this reasonable or am I doing something wrong with
my curve selection?

David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

Vikram:

When defining a screw compressor in eQuest, I see three curves required.

1. EIR f(CHWT & ECT) which is the energy input ratio as a function of the
evaporator leaving water temperature and the condenser water entering
temperature. This is called lift. eQuest calls this DT.

2.Cap f(CHWT & ECT) which is the capacity as a function of lift.

3.EIR f(PLR & DT) which is the energy input ratio as a function of the part load
ratio and the DT or chiller lift.

Creating a performance curve using the temperature difference between the
condenser water entering and leaving temperature is incorrect. I think you
realize this because your chiller energy went up 30%.

If you are modeling a constant chilled water supply temperature you can plug
that in as Independent 1 and use the condenser entering water temperature as
Independent 2. If your chilled water temperature will be reset based on
terminal load or outside air temperature, then you would need to get the chiller
modeled by the manufacturer.

Paul Diglio

Paul Diglio's picture
Offline
Joined: 2011-09-30
Reputation: 400

Vikram,

One of the most common mistakes made when creating chiller curves is not
normalizing them around the ARI rating points. This is required for all
curves except for the new compressor curves.

This is the most helpful section of the documentation that I have found:

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
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

Try normalizing your dependent in the same way shown here and you should get
very different output.

Carol
*
*

cmg750's picture
Offline
Joined: 2010-10-05
Reputation: 0

Aside from the curve input data--wouldn't you expect to see at least a
noticeable increase in chiller energy as Scroll chillers are WAY LESS
Efficient than Screw chillers anwyay?

Vik--if you are staring with a screw chiller profile and then are adjusting
to represent scroll compressor systems I think you would expect to see some
increase in chiller energy for your scroll compressors---and you probably
have multiple scroll compressors where typically the screw chiller profiles
are representing a single compressor unloading performance versus multiple
compressors unloading on one machine...

I agree that maybe 30% increase is out of whack, but my experience tells me
that a 10-15% increase in compressor energy would not be out of line for
scroll compressors and chillers versus screw chiller performance
capabilities.

I'm sorry I can't offer any help with the input of the curve data, my
experience with custom curves is less than ideal (or successful). I have
found that my attempts at using custom chiller curves yields too much
questionability (& confusion on my end) and less than confident energy
results with each manipulation of the eQuest equipment performance curves
and variables.

Pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400

Pasha:

I question you comment that scroll compressors are way less efficient than screw
compressors. Can you explain what constitutes way less?

I looked up two air cooled Carrier chillers, both sized 140 tons.

The screw machine (30XA) has a full load EER of 10.6. The scroll (30RB) has a
full load EER of 9.7.

The screw machine has an IPLV EER of 14.3 and a COP of 4.2. The scroll has a
IPLV EER of 13.6 and a COP of 4.0.

Yes, the screw is slightly more efficient, but I can't see a 10-15% increase in
energy usage especially since the IPLV EERs are so close.

Can you give an example of the EERs you have used for screws v. scrolls?

Paul Diglio

Paul Diglio's picture
Offline
Joined: 2011-09-30
Reputation: 400

Well, maybe I am finally "aging" myself (I didn't think I was getting so
old-school so quickly.)

My past experience with scroll chillers versus screw chillers is that there
'used' to be more of an efficiency difference at the part load values
because of the limitation (or lack thereof) of unloading capability with
scrolls. Screws were the answer that filled in the blanks between recips &
centrifugals originally, and then scrolls filled in at the small tonnages.

So there isn't as much of a difference in the ARI performance values as
maybe I had recalled or conceived in my head, but how do the points look for
the unloading in the scrolls vs. the screws, and for the 140 ton unit you
were looking at how many scrolls were on that unit to make up the tonnage
vs. one/two screws. Do they have 70 ton scroll compressors now?

---which then brings us back to the discussion of how to properly represent
the curve data with multiple compressors----as long as the manufacturer
provides the correct performance data and info for the inputs, then it
should be that easy....

My thoughts were more towards the expected increase in energy use with the
difference in design performance of the compressors themselves. However
the unloading is presented for the *equipment* as a whole unit not for the
individual compressors, and the performance curves are for the whole unit
performance.

...I'm with you on this--just a bit off on my peformance spread with the two
compressors I guess. Maybe I just picked up on a tanget point that was
"mute" to begin with! :)

I welcome any thoughts back.

Pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400