[BLDG-SIM] false load and MIN-RATIO entry
Steven Gates
steve.gates at doe2.com
Thu Jul 28 12:00:42 PDT 2005
Derek,
I'll start by expanding my last explanation to you for the benefit of the
BLDG-SIM community. Then I'll answer your most recent question.
DOE-2.1e and DOE-2.2 chillers are similar in how they use the MIN-RATIO and
the HGB-RATIO (HBG-RATIO is the 2.2 term; 2.1e uses terms like
OPEN-REC-UNL-RAT).
The MIN-RATIO is the part load ratio (PLR) below which the chiller cycles.
For example, if the MIN-RATIO = 0.25 and the PLR is 0.20, then the chiller
runs Frac = 0.20/0.25 = 0.80 of the hour. The PLR assumed in the EIR-FPLR
performance curve when cycling is the MIN-RATIO. The net power consumption
when cycling is then:
OperatingCap = CAPACITY * CapfT(CHWS, ECT)
kW = OperatingCap * ELEC-INPUT-RATIO * EIRfT(CHWS, ECT)
* EIRfPLR(PLR, dT) * Frac / 3413Btu/kW
CHWS = the leaving evaporator temperature
ECT = the entering condenser temperature
PLR = part load ratio, = MIN-RATIO when cycling
dT = ECT-CHWS, only for 2.2, relevant for variable-speed centrifugals
Frac = fraction of the hour the chiller is running
DOE-2.2 makes additional adjustments to the ECT to model variable-flow
water-cooled condensers. It may also increase Frac somewhat to take into
account cycling losses (a modified feature within the last month).
Again note that, when a chiller is cycling, the program assumes that the
chiller runs at the MIN-RATIO when cycled on. This approximates a
controller that does not allow the chiller to load rapidly when restarted
(soft starting), or a controller that cycles closely about the cut-out
temperature.
The HGB-RATIO relevant only for chillers that use hot-gas bypass to continue
running continuously at a PLR at which they otherwise would have to shut
down. Note that most chillers DO NOT use hot gas bypass. This term has
meaning only if the HGB-RATIO is greater than the MIN-RATIO. For example,
assume the HGB-RATIO=0.4, and the MIN-RATIO=0.25:
1) If the actual PLR is 0.5, then the chiller is running continuously with
no hot-gas bypass.
2) If the PLR drops to 0.3, then the chiller is running continuously, but
below the point at which hot-gas bypass initiates. Power consumption
calculated using PLR=0.4 to model the false load due to the hot-gas bypass.
3) If the PLR drops to 0.15, then the chiller is cycling and hot-gas
bypassing simultaneously. The fraction of the hour operating is Frac =
0.15/0.25 = 0.6. When cycled on, the chiller runs at PLR=0.4 to model the
hot-gas bypass.
To answer your follow-up question, when DOE-2.1e allocates load to chillers,
it first loads all chillers equally in proportion to their capacity (i.e.,
the PLR of each enabled chiller is the same). It then checks to see if the
allocation causes any of the equipment to cycle or false load (this implies
that the MIN-RATIO and/or HGB-RATIO of different chillers are markedly
different, which is true in your case). If it detects this condition, then
it adjusts the allocation in an attempt to run all enabled equipment without
cycling or false loading. I believe this is why you are seeing a change in
the loading of the small chiller when the only thing you have done is modify
the MIN-RATIO and hot-gas bypass ratio of the larger chiller.
Does any controller actually do this type of adjustment between chillers? I
am not aware of any that does, although with direct-digital controls any
sequence that can be programmed is possible. In addition to redistributing
load, the controller should ideally also redistribute flow so that the
leaving chilled-water temperature is the same for each chiller (assuming
chillers are in parallel). In DOE-2.2 we eliminated this adjustment because
we did not think it reflected the real world.
Steve G.
-----Original Message-----
From: Derek Whitehead [mailto:Derek.Whitehead at keen.ca]
Sent: Thursday, July 28, 2005 10:53 AM
To: steve.gates at doe2.com
Subject: RE: [BLDG-SIM] false load and MIN-RATIO entry
Thanks very much for your help. The keyword turned out to be
OPEN-REC-UNL-RAT. Unfortunately, solving this problem created another,
which I hope you'll be able to help me with.
I am modelling two chillers: one small double bundle chiller and one larger
reciprocating chiller. I have sequenced the operation so that the smaller
chiller is the only one to operate when the load is at or below its max
capacity. When this capacity is exceeded, both chillers operate, with the
smaller chiller sequencing on before the larger chiller.
Now for the problem. When I change the minimum operating ratio and hot gas
bypass ratio of the larger chiller from 25% to 17%, the PS-C report shows
the number of hours operating in each load range changes for the smaller
chiller. I would have thought that this should be unchanged. Any thoughts?
>>> "Steven Gates" <steve.gates at doe2.com> 07/28/05 8:28 am >>>
If MIN-RATIO is less than the HGB-RATIO (I do not recall the exact name of
this keyword), then hot-gas bypass will be simulate whenever the PLR is
between the MIN-RATIO and the HGB-RATIO.
-----Original Message-----
From: BLDG-SIM at GARD.COM [mailto:BLDG-SIM at GARD.COM] On Behalf Of Derek
Whitehead
Sent: Wednesday, July 27, 2005 4:24 PM
To: BLDG-SIM at GARD.COM
Subject: [BLDG-SIM] false load and MIN-RATIO entry
I am modelling an open reciprocating chiller in DOE2.1e. When I specify a
MIN-RATIO lower than the default value of 0.25, the PS-C report shows a
false load on the chiller. When I specify a value of 0.25, no false load is
reported. This suggests to me that hot gas bypass is being modelled.
The reference manual says that, if the item of equipment operates with a
demand that is more than zero but less than the MIN-RATIO, the chiller will
cycle on and off. Could someone please tell me what I'm missing? Thanks.
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