[Equest-users] Condenser loop control?
Carol Gardner
cmg750 at gmail.com
Fri Nov 19 10:21:00 PST 2010
The commands you need are under the condenser water (CW) loop. If you click
on the CW loop, and then on the Controls tab, you will see the Setpoint
Control box which defaults to fixed. When you look at the pull down menu,
however, you will see the option to select Load Reset. Load Reset, when
accessed from the CW loop, allows the tower to be controlled using wet bulb
reset. The way that the developers set it up, as described below, is to get
the best of both worlds (thanks, Steve!), savings from cooling tower VFDs
first and wet bulb reset second.
Volume 2: Dictionary <volume2dictionary.htm> > HVAC
Components<hvaccomponents.htm>>
HEAT-REJECTION <heatrejection1.htm>
Temperature Control in Cooling Towers
The supply temperature that a tower will attempt to achieve is set by the
attached CIRCULATION-LOOP through the COOL-SETPT-CTRL, COOL-SETPT-T, COOL-
RESET-SCH, COOL-SETPT-SCH, and LOOP-SETPT-RNG keywords (see CIRCULATION-LOOP
command). For open towers and fluid coolers only there is one additional
temperature control mode available that tries to achieve an optimal
compromise between tower fan energy and cooling compressor energy. You
activate this sequence by with COOL-SETPT-CTRL = LOAD-RESET in the attached
CIRCULATION-LOOP. The sequence is as follows:
· The chiller load is determined on an hourly basis. In the field, this
would be accomplished either by a monitoring a signal directly from the
chiller control panel, or, perhaps more commonly, by measuring the
temperature rise across the chiller condenser, and comparing it to the
design temperature rise. For example, if the design condenser temperature
rise is 10F (5.6K), and the actual rise is 6F (3.3K), then the chiller
loadis assumed to be 60%. (This assumes that the condenser is constant
flow.)
· The tower air flow (and fan speed) required to reject a given chiller
load is essentially linear with load (open towers) or drops off faster than
the load (fluid coolers). Rather than controlling a variable-speed tower fan
on the basis of leaving tower temperature as is conventionally done, this
algorithm will vary the tower fan speed directly on the basis of chiller
load. For the example given above, the tower fan will run at 60% speed when
the chiller load is 60%. The leaving tower temperature then floats with both
wet-bulb temperature and chiller load. This concept gives priority to
minimizing tower energy consumption, but still achieves chiller energy
savings as the leaving tower temperature floats.
· To further increase chiller efficiency, it can be recognized that the
tower fan power varies approximately as the cube of the air flow (and load),
and that the majority of the energy savings are achieved when the fan is
close to full speed. Once the fan speed has been reduced to 70% or so,
additional tower fan energy savings will be negligible compared to the
potential chiller savings that might be achieved by reducing the approach to
the wet-bulb.
· Keywords are provided to allow you to experiment with this concept.
For example, at 50% load the default fan speed will be 50%. You might want
to investigate what happens if the tower fan is not allowed to drop below
65% speed at 40% load.
· Maximum and minimum condenser temperature limits are included. For
example, you might not want the leaving tower temperature to ever exceed 85F
(29.4C) or to drop below 65F (18.3C). The fan speed algorithm described in
(2) and (3) will be overridden to ensure that these limits are not exceeded.
· Two-speed tower fans use a variation of this algorithm. As before,
the tower will cycle between low and high speed to maintain the tower
setpoint. However when the leaving temperature drops below the setpoint, the
fan will not cycle between off and low speeds. Instead, the fan will remain
on low speed unless the leaving temperature drops all the way down to
MIN-TWR-WTR-T, at which point it will cycle between off and low to prevent
the tower temperature from dropping any further.
This control sequence is not applicable to single-speed fans. It applies to
both CW loops as well as WLHP loops. Maximum energy savings might be
achieved for WLHP loops, as these systems typically have a lower cooling COP
than chillers, and the heat rejection commonly uses a fluid cooler which has
a higher fan horsepower than an equivalent open tower.
When using the LOAD-RESET control sequence, you can try experimenting with
the values of the MIN-RESET-PLR, MIN-VFD-SPEED, MAX-RESET-SPEED to optimize
the energy efficiency of the system.
The LOAD-RESET tower keywords are illustrated in Figure 36 and are as
follows:
------------------------------
Subtopic help for this Topic:
MIN-RESET-PLR
MIN-VFD-SPEED
MAX-RESET-SPEED
CIRCULATION-LOOP:COOL-SETPT-T
CIRCULATION-LOOP:MIN-RESET-T
On Fri, Nov 19, 2010 at 9:33 AM, Bishop, Bill <wbishop at pathfinder-ea.com>wrote:
> There is even more information in DOE-2 Help under Volume 2: Dictionary >
> HVAC Components > HEAT-REJECTION > Temperature Control in Cooling Towers. However,
> I don’t see any commands that let you control the loop or tower using
> wet-bulb temp, only the dry-bulb temp using OA-RESET.
>
>
>
> Bill
>
>
>
> *William** Bishop, PE**, BEMP, LEED® AP **|** **Pathfinder Engineers &
> Architects LLP***
>
> Mechanical Engineer
>
>
>
> 134 South Fitzhugh Street Rochester, NY 14608
> T: (585) 325-6004 Ext. 114 F: (585) 325-6005
>
> wbishop at pathfinder-ea.com www.pathfinder-ea.com
>
> P Sustainability – the forest AND the trees. P
> ------------------------------
>
> *From:* equest-users-bounces at lists.onebuilding.org [mailto:
> equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Carol Gardner
> *Sent:* Friday, November 19, 2010 11:53 AM
> *To:* Chris Jones
> *Cc:* equest-users at lists.onebuilding.org
> *Subject:* Re: [Equest-users] Condenser loop control?
>
>
>
> The commands you are looking for are for the cooling tower. See below.
>
> Carol
>
>
>
> Volume 2: Dictionary <http://volume2dictionary.htm> > HVAC Components<http://hvaccomponents.htm>>
> CIRCULATION-LOOP <http://circulationloop2.htm> > Loop Temperature Control<http://looptemperaturecontrol.htm>
> *COOL-SETPT-CTRL*
>
> Code word specifies the loop temperature control sequence:
>
> FIXED specifies that the loop supply temperature is
> controlled to a fixed value, specified by the COOL-SEPT-T
>
> OA-RESET specifies that the loop supply temperature is
> reset on outdoor air according to the COOL-RESET-SCH.
>
> SCHEDULED specifies that the loop supply temperature is
> controlled to a schedule specified by the COOL-SETPT-SCH.
>
> LOAD-RESET specifies that the loop supply temperature is
> reset so that the valve of the worst-case coil is fully open. The minimum
> reset temperature for a cooling-type loop is MIN-RESET-T. LOAD-RESET is
> applicable to any loop serving a chilled-water coil (CHW, PIPE2). It is also
> applicable to a condenser-water loop; here this control mode will enable the
> cooling tower temperature to float with the load and wet-bulb temperature
> as described in the cooling tower sections.
>
> Since a WLHP loop also uses a cooling
> tower, LOAD-RESET is applicable to the heat-rejection side of a WLHP loop.
> It does not apply, however, to the heating setpoint of a WLHP loop.
>
> This mode maximizes the efficiency of the
> primary equipment and minimizes the loop's thermal losses (but at the
> expense of pumping energy in a variable-flow loop). Refer to LOOP-FLOW-
> RESET for information on how to minimize the pumping penalties associated
> with this control scheme.
>
>
>
> On Fri, Nov 19, 2010 at 5:26 AM, Chris Jones <cj at enersave.ca> wrote:
>
> I believe that the CONT-SETPT-CTRL = VARIABLE provides similar control?
>
>
>
> At 08:16 AM 19/11/2010, Brian Fountain wrote:
>
> In DOE-2.1e, you had the option of condenser loop control by wet-bulb
> reset. I cannot figure out how to do that in DOE-2.2. Any suggestions or
> was this option lost in the combining of systems & plants.
>
>
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>
> Chris Jones
> 14 Oneida Avenue
> Toronto, ON M5J 2E3.
> Tel. 416-203-7465
> Fax. 416-946-1005
>
>
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>
>
>
> --
> Carol Gardner PE
>
--
Carol Gardner PE
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