RESETTING VAV DUCT STATIC
PRESSURE SET POINT
Steve Taylor’s ASHRAE Journal Column of
November 2015 presents details for direct digital
control (DDC) of VAV fans to meet Standard 90.1’s
requirement for resetting duct static pressure set
point. This paper will focus on the potential energy
reductions possible with duct pressure reset
control.
SYSTEM ENERGY EQUILIBRIUM
(SEE) MODELING
The objective of a System Energy Equilibrium (SEE)
building energy model is to duplicate the hourly
performance of a real building at all operating
conditions of weather and load; giving flows,
temperatures, cooling loads, kW demand of
equipment, and total site kW as weather and
operational conditions change. The (SEE) model, as
presented here, consists of a set of simultaneous
equations that obey the laws of thermodynamics,
models the hour by hour loads of the building and
the response of the central chilled water system
(CCWS) to the building loads, and includes the
nonlinear performance characteristics of the plant
equipment and air side equipment. A (SEE) model
iterates to steady state energy equilibrium after a
perturbation to the system just as a real system
responds; a defining characteristic of a (SEE)
model.
The primary objective of this paper is to
demonstrate the energy effect of VAV fan duct
pressure reset control. Where disagreement with
the (SEE) model answers exists a discussion sidebar
may be provided with input by reviewers and
response by the author.
THE BUILDING & ASSUMED WEATHER
The building selected for this study is defined by
the Pacific Northwest National Laboratory (PNNL)
study of standard 90.1-20101, a large 13 story St
Louis office building, Figure 1, with 498,600 square
feet of air conditioned space. A link to the (PNNL)
study is given by the reference1. The building
Kirby Nelson PE 2/9/2016
schedules and other details of the building, as
defined by the (PNNL) study, are in this model
design but the plant of this study is designed to a
series of articles in the ASHRAE Journal, Taylor
20112. Figure 2 shows the assume weather
conditions for the 24 hours to be modeled. The
peak building load occurs at 4PM with 100% solar,
99.8F dry bulb and 77.2F wet bulb.
The building is modeled with an internal zone that
has all electrical and people loads plus the roof
load and a perimeter zone that models all
wall/glass solar and transmission loads plus air
infiltration or exfiltration.
Visit http://kirbynelsonpe.com for other papers.
MODEL CHARACTERISTICS/LIMITATIONS
The model presented here has the following known
limitations. All air handlers of the system are assumed
to be of the same size and model and to be equally
loaded. The chiller/towers are assumed to be of the
same size and model and equally loaded. The model is
of the total building and not a model of individual
spaces within the building. Therefore the fresh air,
infiltration, and exhaust, is for the building total.
Thermostat set points are assumed to be the same for
all interior and/or perimeter spaces. To eliminate any of
these limitations requires more computer power than
the author presently has available.
FIG. 1 Building
Page 1
FIG. 2 Weather conditions for assumed 24 hours peak
design day
AIR SIDE SCHEMATIC
A brief discussion of the schematic; the building
(yellow) has an internal load of 300.8 ton and the
perimeter load is 156.1 ton. The return air of
280,361 CFM is pulled/pushed to the inlet of the
VAV fans (blue) where 48,628 CFM is exhausted
and 231,733 CFM is pulled into the VAV fans. Fresh
air of 41,817 CFM (blue) is pulled in by the VAV
fans for a total of 273,551 CFM (red) delivered to
the coils by 273 kW of VAV fan power. (dh) = 5.568
& fan efficiency of .657 is modeled to reference
one. Fifty five degree air (F) exists the coils (gray)
presenting a plant load of 822 ton and air flow of
176,985 CFM to the interior duct system and
96,565 CFM to the perimeter duct system (green).
The terminal fans add heat to the air therefore
56.11F air is delivered to the building interior and
57.04F air to the perimeter to meet the respective
building loads. The building kW is 731.4, the fan kW
479.1 and the total air side or site kW is 1210.5 as
shown on the schematic (gray).
Figure 3 is the schematic of the air side system at
4PM peak design day conditions. The values on the
schematic are at energy equilibrium and obey the
laws of thermodynamics, nomenclature is given at
the end of this paper. All values on the schematic
are part of the model equations/model.
Kirby Nelson PE 2/9/2016
Page 2
2
BLD ft = 498600
%clear sky = 100.0%
InfilLat-ton = 30.84
Tdry-bulb = 99.8
# floors = 13
2
Roof ft = 38,354
2
N/S wall ft = 40,560
Infil-CFM = 6811 <
Twet-bulb= 77.2
Infilsen-ton =
WallNtrans ton=
4.92
E/W wall ft = 27,008
WallStrans ton=
5.30
Wall % glass= 37.5%
WallEtrans ton=
4.09
2
Glass U = 0.55
WallWtranston=
3.28
Wall U = 0.09
GlassN trans ton =
17.29
Glass SHGC = 0.40
Wall emitt = 0.55
RoofTrans ton = 33.3
Roofsky lite ton = 0.0
Peopleton = 60
plugton&kW = 93
GlassS trans ton =
GlassE-trans ton =
17.29
11.51
GlassW-trans ton =
11.51
7.1
GlassN-solar-ton =
Lightton&kW= 115
kW
GlassS-solar-ton =
20.8
328
GlassE-solar ton =
404
GlassW-solar ton =
4.7
33.1
BLD kW=
FAN kW=
731.4
479.1
Total Bldint-ton = 300.8
(int-cfm)to-per-ret= 176985
Tstat-int= 75.0
SITE kW =
(Bld)int-air-ton= -300.8
^
^
(fan)int-ter ton&kW=
kW
17.7
62.4
GlassTot-solar-ton =
65.7
Tstat-per =
75.0
return
^ (Bld)per-air-ton= -156.1
air
Tair supply per= 57.04
Design
ABS Bld Ton =
Ton
57.6
(int cfm)per-ton = 0.00 >
Tot Bldper-sen-ton = 156.1
v
4PM
ASHRAE
17.6
GlassTot-trans-ton=
1210.5
Design
Tair supply int= 56.11
WallTot trans ton =
15.2
456.91
^
(fan)per-ter ton&kW=
Ton
kW
17.7
62.4
0.0
0.0
V
Theat-air= 55.0
(D)heat ton&kW =
Treheat air = 55.0
(D)reheat ton&kW =
(D)int-air-ton= -318.6
Tair coils = 55.00
(coil)H2O-ft/sec=
1.05
(coil)des-ft/sec=
1.20
LMTD=
14.52
(COIL)L+s-ton=
822
-173.8
Tair coils= 55.00
(D)per-CFM= 96,565
duct
^
(coil)gpm=
UAdesign=
^
COIL
^
^
2.46
31.62
^
(H)coil=
1.6
(H)coil-des=
2.1
^
26 F.A.Inlet
42
^
26 VAV FANS
TFA to VAV =
99.8
> Tret+FA = 79.57
>(FA)sen-ton = >
168.6
(dh) = 5.568
> (FA)CFM=
> (FA)Lat-ton=
(FA)kW=
41,817
114.1
0.0
^
38.4
2.66
UA=
(FAN)ton-VAV= 77.5
(FAN)kW-VAV= 273
statFA=
duct
(one coil)ton=
Tair VAV= 82.72
(FAN)VAV-CFM= 273,551
<<<<
0.0
62.4
Peri
(D)per-air-ton=
Interior
(D)int-CFM= 176,985
>>>(Coil)sen-ton=
682
(coil)cap-ton=
35.7
0.0
^
V
TBLD-AR = 75.00
(Air)ret-CFM =
280,361 Return
(FAN)ret-kW=
82
(FAN)ret-ton=
23.2
(Air)ret-ton =
527.9
V
Tar-to-VAV = 75.92
VAVret-ton = 436.3
InfilVAV-Lat-ton = 25.49
< VAVret-CFM = 231,733 <
> Efan-VSD= 0.657
VAV inlet-sen-ton = 604.9
VAVinlet-lat-ton= 139.6
Fan
V
ExLat-ton = -5.3
ExCFM = -48,628
SEE SCHEMATIC
air side
TEx = 75.92
Air temp green
kW red
Exsen-ton = -91.6
Air CFM purple
Ton blue
V
v
FIG. 3 Air Side Schematic at peak conditions. VAV fan
kW = 273. (dh)=5.568in
Kirby Nelson PE 2/9/2016
Page 3
Figures 4 & 4A give the VAV fan performance with and
without duct pressure control. CFM of air and supply air
temperatures are the same for both conditions of duct
pressure control. The difference is in VAV fan kW where
the kW increases at off peak hours, see primary
horizontal axis, with constant duct pressure control of
Figure 4A.
(Duct)interior-CFM
(Temp)air-supply-interior
(Duct)perimeter-CFM
(Temp)air-supply-perimeter
FAN VAV-CFM
180,000
159,514
160,000
70
176,985
100,000
66
61.9
62.7
63.3
64
96,565
61.0
80,000
61.4
73,970
58.6
61,363
57.7
59.8 60.3 60.3 59.9
57.3 57.0
40,752
58.2
57.6
40,000
57.0
56.9
56.2 56.2 56.2 56.1
20,000
30,832
60,000
0
62
60
58
AIR TEMP. (F)
(CFM)
120,000
FIG. 5 Fan system kW with duct pressure control
68
140,000
56
54
52
FAN kW-VAV
(ASHRAE Design)SUPPLY AIR TEMP-CFM & VAV kW-CFM
FIG. 4 VAV fan CFM & air temp with duct pressure
control
FIG. 5A Fan system kW-5.5in constant duct pressure.
Figures 5 & 5A give the fan system kW for both
conditions of duct pressure control. Duct heat & reheat
are zero for these conditions of weather but will
become significant with winter conditions, a study we
may add in the future. Figure 5A illustrates the VAV fan
kW is the big increase with the return fan also
increasing a lesser amount due to the constant duct
static pressure. The increased fan system kW results in
an increase load on the plant as shown next.
FIG. 4A VAV fan CFM & air temp with constant 5.5in
duct pressure
Kirby Nelson PE 2/9/2016
Page 4
FIG. 6 VAV fan kW and plant load with duct static
pressure control
FIG. 6A VAV fan kW & plant load with constant 5.5in
duct pressure.
Figures 6 & 6A show the difference in plant load due to
the constant duct pressure. The plant load is the same
at peak conditions, 4PM 822 ton, but increases as
shown by the primary horizontal axis of the two figures.
The secondary horizontal axis gives the duct pressure
control values. The increased plant load increases the
plant kW, to be shown below, but first let’s look at the
increase in the air temperature of the constant duct
pressure (6A) as it leaves the VAV fan and enters the
coils.
Kirby Nelson PE 2/9/2016
FIG. 7 VAV fan performance with duct pressure control
FIG. 7A VAV fan performance with constant 5.5in duct
pressure
Figure 7 shows the temperature of air to the coils
increases with increased VAV fan kW, as required by the
first law, and Figure 7A illustrates the difference in air
temperature and VAV fan kW due to the increased VAV
fan kW as a result of constant duct pressure. The
primary horizontal axis shows the VAV fan efficiency is
essentially the same for both conditions of duct
pressure and the secondary horizontal axis illustrates
the difference in duct pressure. The next figures will
look at total system kW demand and therefore energy
saving due to resetting duct pressure.
Page 5
Note that this is based on the dh=5.568in of Figure 3
peak conditions. A better investment might be in
reducing the peak dh=5.568 with a better duct system;
perhaps round duct? The next two charts investigate.
BLD sq-ft = 498,600
ALL ELECTRIC Peak day
Design
24hr
BLD.24hr-kW=
10,096
(Fan)24hr-kW =
8,263
(Duct)24hr-heat kW= 0
(FA)24hr-heat kW= 0
Heat24hr-total kW=
0
Plant24hr-kW=
7,683
SYST 24hr-kW =
26,043
FIG. 8 System kW with duct pressure control
(CCWS)24hr-kW=
BLD.24hr-kW=
Total24hr-kW =
Weather24h-Ein-ton=
SITE24h-kW-Ein-ton =
Plant24h-kW-Ein-ton =
Total24h-Ein-ton =
Pump24hr-heat-ton =
AHU Ex24hr-Lat-ton =
AHU Ex24hr-sen-ton =
Tower24hr-ton-Ex =
Total E24hr-out-ton =
ASHRAE
15,946
10,096
26,043
6923
5222
2185
14330
-80
-110
-1265
-12875
-14330
Design
FIG. 8A System kW with constant 5.5in duct pressure.
Comparing figures 8 & 8A illustrates the building kW is
the same for both, as required, but the plant kW and air
handler kW increase as shown (8A) for all hours except
at 4PM peak conditions. The next figures sum these kW
values so we can get a handle on the 24 hour energy
consumption reduction offered by duct pressure reset
control.
The two charts below illustrate the 24 hour energy
consumption for the two conditions of duct pressure
control and also gives the energy in = energy out for the
two conditions. Over the 24 hours as defined by Figure
2, the constant duct pressure of 5.5in, right chart,
results in 2003kWh greater consumption or an 8.3%
increase over the duck pressure control of the left chart.
Kirby Nelson PE 2/9/2016
Page 6
on the zone requiring the most pressure; a complicated
control system. This analysis finds about 8.3% reduction
in kW demand at peak design day conditions for an
office building in St Louis. The % savings will decrease
for moderate summer and spring/fall weather; how the
system will react to winter conditions is of interest to
the author and may be included at a later date.
Designing a duct system with less static pressure or
resetting duct pressure based on time of day or outside
temperature seems less likely to result in a visit of
Newton’s 5th law, see below. Operating at constant
duct pressure with no reset is the safe mode but costs
8.3% to about 2% (estimate) more energy; perhaps the
best design in some cases?
The two charts above illustrate the effect of reducing
the duct pressure to 4.53in at peak design with a more
efficient duct design. The left charts gives the 24 hour
result with duct pressure control and the right chart is
with constant 4.53in duct pressure. The difference is
now about 6.5% more energy over the 24 hours at peak
design day conditions, down from 8.3% from above. The
% energy savings will continue to decrease as the duct
pressure at design conditions decreases. Also the
difference will decrease during typical summer and
spring/fall weather conditions, however I am not sure
how the difference will be during winter weather,
perhaps a future study?
Conclusions
MY final conclusion is that this issue needs more tests
and (SEE) model analysis.
Newton’s 5th law, (If anything can possible go
wrong it will) and Newton’s 5th law brings to mind
the KISS principle.
HOME WORK
For those of you who would like to study the action of
the office CCWS in more detail I have included the
following three figures. The first is the air side
schematic at 10AM and the second is the plant
schematic at 10AM. The third is a chart of the CFM and
air temperatures of the system over 24 hours.
NOMENCLATURE given below
For the given building and weather I have given a
quantitative analysis of Taylor’s ASHRAE Journal article
on duct pressure control reset. Taylor points out that
STD. 90.1 requires VAV duct static pressure reset based
Kirby Nelson PE 2/9/2016
Page 7
2
BLD ft = 498600
%clear sky = 100.0%
Tdry-bulb = 87.0
# floors = 13
2
Roof ft = 38,354
2
N/S wall ft = 40,560
InfilLat-ton = 27.50
infil-CFM = 6811 <<
Infilsen-ton = 7.4
Twet-bulb= 73.0
WallNtrans ton=
2.28
E/W wall ft = 27,008
WallStrans ton=
2.28
Wall % glass= 37.5%
WallEtrans ton=
2.00
Glass U = 0.55
WallWtranston=
1.52
Wall U = 0.09
GlassN trans ton =
8.37
Glass SHGC = 0.40
GlassS trans ton =
8.37
Wall emitt = 0.55
RoofTrans ton = 1.8
Roofsky lite ton = 0.0
GlassE-trans ton =
5.57
GlassW-trans ton =
5.57
GlassN-solar-ton =
6.1
2
Peopleton = 60
plugton&kW = 93
Lightton&kW= 115
Total Bldint-ton = 269.4
(int-cfm)to-per-ret= 159514
kW
GlassS-solar-ton =
6.6
328
GlassE-solar ton =
55.4
404
GlassW-solar ton =
Tstat-int= 75.0
(Bld)int-air-ton= -269.4
Tair supply int= 56.24
^
(fan)int-ter ton&kW=
4.1
BLD kW=
^
731.4
FAN kW=
372.6
SITE kW =
1104.0
Design
Design
ABS Bld Ton =
Ton
kW
17.7
62.4
GlassTot-trans-ton=
8.1
27.9
GlassTot-solar-ton =
72.1
(int cfm)per-ton = 0.00 >
Tot Bldper-sen-ton = 115.4
v
Tstat-per = 75.0 return
^ (Bld)per-air-ton= -115.4
Tair supply per= 57.67
10AM
ASHRAE
WallTot trans ton =
384.79
air
^
fanper-ter ton&kW=
Ton
kW
17.7
62.4
0.0
0.0
V
Theat-air= 55.0
(D)heat ton&kW =
Treheat air = 55.0
(D)reheat ton&kW =
0.0
0.0
62.4
(D)int-air-ton= -287.1
533
(coil)cap-ton=
(coil)H2O-ft/sec=
28.6
(coil)des-ft/sec=
1.20
LMTD=
13.43
(COIL)L+s-ton=
648
^
COIL
^
^
UA=
(one coil)ton=
(H)coil=
(H)coil-des=
^
Tair VAV= 80.35
<<<<
(FAN)VAV-CFM= 233,484
57
(FAN)ret-ton=
16.3
(Air)ret-ton =
448.8
^
26 VAV FANS
> Tret+FA = 77.77
(dh) = 4.432
(FA)kW=
0.0
TBLD-AR = 75.00
(FAN)kW-VAV= 191
87.0
41,817
V
2.1
240,295 Return
120.4
93.6
1.0
(FAN)ret-kW=
TFA to VAV =
> (FA)CFM=
2.13
24.93
(FAN)ton-VAV= 54.2
>(FA)sen-ton = >
> (FA)Lat-ton=
^
^
(Air)ret-CFM =
^
26 F.A.Inlet
statFA=
42
Peri
duct
(D)per-CFM= 73,970
(coil)gpm=
30.2
UAdesign=
2.66
^
0.83
-133.1
Tair coils= 55.00
duct
(D)int-CFM= 159,514
>>>(Coil)sen-ton=
(D)per-air-ton=
Interior
Tair coils = 55.00
V
Tar-to-VAV = 75.75
VAVret-ton = 358.0
InfilVAV-Lat-ton = 21.93
< VAVret-CFM = 191,667 <
> Efan-VSD= 0.638
VAVinlet-sen-ton= 478.4
VAVinlet-lat-ton= 115.5
Fan
V
ExLat-ton = -5.6
ExCFM = -48,628
SEE SCHEMATIC
air side
TEx = 75.75
Air temp green
kW red
Exsen-ton = -90.8
Air CFM purple
Ton blue
Kirby Nelson PE 2/9/2016
V
v
Page 8
Condenser
(cond)ton= 382
Pipesize-in =
Tower
TCR= 92.2
> gpmT= 1800
> (ewt)T= 90.8
tfan-kW=
8.3
TCR-app= 1.42
(H)T-total= 66.4
(H)T-static = 9.9
Tfan-kW=
16.6
Trange= 10.2
tfan-% =
100%
(lwt)T = 80.6
Tapproach = 7.6
tton-ex=
-385
(COND)ton= 765
PT-heat = -1.31
(H)cond= 43.0
(cond)ft/sec= 9.7
< pT-kW= 27.1
ASHRAE
St Louis
(chiller)% = 73%
(chiller)#= 2
(CHILLER)kW= 363
(chiller)kW/ton= 0.554
Plant kW =
<
EfTpump= 0.83
Ptower # = 2
Compressor
(chiller)kW= 181
(chiller)lift= 50.1
>
(H)T-pipe= 13.5
6"
T #=
2
T-Ton-ex=
-769
Trg+app =
17.8
Design
90.1-2010
Large
Office
Peak day
Weather
Design
%clear sky =
10AM
100%
conditions
Tdry bulb =
Twet bulb =
87.0
73.0
440.1
Evaporator
(evap)ton= 327.6
TER= 42.1
TER-app= 1.95
^
EVAPton= 655
(H)evap= 34.5
(evap)ft/sec= 8.38
(evap)des-ft/sec= 8.38
^
V
gpmevap= 1200
Psec-heat-ton =
(lwt)evap = 44.07
^
(H)pri-total= 44.3
(H)pri-pipe= 2.5
(H)pri-fitings= 7.0
Tbp=
(Ef)c-pump= 0.81
Pc-heat-ton= -0.67
^
< pc-kW= 12.4
v
44.07
gpmbp=
-414
0.30
(H)pri-bp=
v
(ewt)evap =
-1.80
Psec-kW= 21.2
Efdes-sec-p = 0.80
Efsec-pump = 0.70
>
57.17
> (ewt)coil= 44.1
(H)sec= 100.6
40
PLANTton = 648
(H)sec-bp= 0.00
Pipesize-in = 8.0
(H)sec-pipe=
< (gpm)sec=
786
< (lwt)coil= 64.1
Pchiller-# = 2
St Louis
Performance
10AM
10AM
chillerkW/evapton= 0.554
All Electric
Fuel Heat
kW
THERM
Design
BLD.kW=
731.4
plantkW/site ton= 0.679
CCWSkW/bld ton= 2.11
Ductheat=
0.0
WeatherEin-ton = 430.5
(FA)heat=
0.0
(Site)kW-Ein-ton = 314.0
Heat total =
0.0
PlantkW-Ein-ton = 125.2
PlantkW=
440.1
Total Ein-ton = 870
SystkW =
1544.1
Pumptot-heat-ton =
-3.8
AHU ExLat-ton =
-5.6
(Fan)kW =
BLD.kW=
372.6
0.00
0.00
731.4
0.00
Plant
1544.1
SEE
SCHEMATIC
Ton
Blue
kW
Red
AHU Exsen-ton = -90.8 CCWSkW =
812.7
Water temp pink
Tower Tton-Ex =
-769
1544.1
Water gpm orange
Total Eout-ton =
-870
Kirby Nelson PE 2/9/2016
SystkW =
air
temp green
Page 9
(FAN)VAV-CFM
6,239
33,674
11,225
41,817
41,817
41,817
499
41,817
280,000
273,551
260,000
233,484
240,000
220,000
231,733
200,000
180,000
191,667
78.04
82.72
160,000
77.76
80.35
140,000
120,000
92,195
75.92
75.75
75.57
100,000
80,000 69,330
60,740
85,956
60,000
40,000
20,000 1,125 499 499
0
174 157 151 174 392 648 695 755 822 590 372 241
95
90
85
80
75
70
AIR TEMPERATURE (F)
(FA)-CFM
(Temp)air-VAV to coil
26 Coils sensible ton
32,422
(CFM)
(VAV)ret-CFM
(Temp)air ret-to-VAV
65
60
Plant load = 26 Coils latent + sensible ton
(ASHRAE Design)CFM & AIR Temp TO COIL & COIL LOADS
References
1. Liu, B. May 2011. “Achieving the 30% Goal:
Energy and Cost Savings Analysis of ASHRAE
Standard 90.1-2010” Pacific Northwest National
Laboratory.
http://www.energycodes.gov/achieving-30-goalenergy-and-cost-savings-analysis-ashraestandard-901-2010
2.Taylor, S. 2011. “Optimizing Design & Control of
Chilled Water Plants.” ASHRAE Journal (12)
NOMENCLATURE
Each of the more than 100 variables of the system
defined.
Building structure;
BLD ft2 = air conditioned space
# Floors = number of building floors
Roof ft2 = roof square feet
N/S wall ft2 =north/south wall square feet
E/W wall ft2 =east/west wall square feet
Wall % glass = percent of each wall that is glass
Glass U = glass heat transfer coefficient
Wall U = wall heat transfer coefficient
Glass SHGC = glass solar heat gain coefficient
Wall emit = wall solar index
Building interior space;
Rooftrans-ton =transmission through roof (ton)
Roofsky-lite-ton =sky lite load (ton)
Peopleton = cooling load due to people (ton)
Plugton&kW = cooling load & kW due to plug loads
Lightton&kW = cooling load & kW due to lights
Total Bldint-ton = total building interior load (ton)
Kirby Nelson PE 2/9/2016
(int-cfm) to-per-return = CFM of interior supply air that
returns to perimeter of building
Tstat-int = interior stat set temperature (F)
Bldint-air-ton = supply air ton to offset interior load
BLD kW = total building kW demand
Building perimeter space;
%clear sky = percent clear sky
Tdry bulb = outside dry bulb temperature (F)
Twet bulb = outside wet bulb temperature (F)
Infillat-ton = latent load due to air infiltration (ton)
InfilCFM = air infiltration CFM
ExfilCFM = air exfiltration CFM
Infilsen-ton = sensible load due to air infiltration (ton)
Enfilsen-ton =sensible load due to air exfiltration (ton)
Walln trans ton = north wall transmission (ton)
Walls trans ton = south wall transmission (ton)
WallE trans ton = east wall transmission (ton)
Wallw trans ton = west wall transmission (ton)
Walltot-trans-ton = total wall transmission (ton)
GlassN-trans-ton = north wall glass transmission (ton)
GlassS-trans-ton = south wall glass transmission (ton)
GlassE-trans-ton = east wall glass transmission (ton)
GlassW trans-ton = west wall glass transmission (ton)
Glasstot-trans-ton = total transmission thru glass (ton)
GlassN-solar-ton = north glass solar load (ton)
GlassS-solar-ton = south glass solar load (ton)
GlassE-solar-ton = east glass solar load (ton)
GlassW-solar-ton = west glass solar load (ton)
Glasstot-solar-ton = total glass solar load (ton)
(int cfm)per-ton = effect of interior CFM to wall (ton)
Total Bldper-sen-ton total perimeter sensible load (ton)
Tstat-per = perimeter stat set temperature (F)
Bldper-air-ton = supply air ton to offset perimeter load
Air handler duct system
Interior duct
Tair supply int = temp air supply to building interior (F)
(fan)int ter ton&kW = interior ton & kW due to terminal
fans
(D)int-air-ton = cooling (ton) to building interior duct
Tair coils = supply air temperature off coils to duct (F)
(D)int-CFM = supply air CFM to building interior duct
Perimeter duct
Tair supply per =temp (F) air supply to building
perimeter
(fan)per ter ton&kW = perimeter ton & kW of terminal
fans
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Theat-air = temp supply air before terminal fan heat
(F)
(D)heat-ton&kW = heat to perimeter supply air ton &
kW
Treheat air = temp perimeter supply air after reheat
(F)
(D)reheat ton&kW = reheat of perimeter supply air ton
& kW
(D)per-air-ton = cooling (ton) to perimeter duct
Tair coils = supply air temperature off coils to duct (F)
(D)per-CFM = supply air CFM to perimeter duct
(ABS Bld Ton) = absolute building load on (CCWS)
Coil
(coil)sen-ton = sensible load on all coils (ton)
(coil)cap-ton = LMTD * UA = capacity (ton) on one coil
(coil)H2O-ft/sec = water velocity thru coil (ft/sec)
(coil)design-ft/sec = coil design water velocity (ft/sec)
LMTD = coil log mean temperature difference (F)
(coil)L+s-ton = latent + sensible load on all coils (ton)
(coil)gpm = water flow (gpm) thru one coil
UAdesign = coil UA design value
UA = coil heat transfer coefficient * coil area. UA
varies as a function water velocity (coil)gpm thru the
coil, as the (coil)gpm decreases the coil capacity
decreases.
(one coil)ton = load (ton) on one coil
(H)coil = air pressure drop thru coil (inches)
(H)coil-design = design air pressure drop (inches)
VAV Fan system
Fresh air
statFA = fresh air freeze stat set temperature (F)
TFA to VAV = temperature of fresh air to VAV fan
(FA)sen-ton = fresh air sensible load (ton)
(FA)CFM = CFM fresh air to VAV fan inlet
(FA)Lat-ton = fresh air latent load (ton)
(FA)kW = heat kW to statFA set temperature
Air return
TBLD-AR = return air temp (F) before return fans
(Air)ret-CFM = CFM air return from building
(FAN)ret-kW = return fans total kW
(FAN)ret-ton = cooling load (ton) due to (FAN)ret-kW
(Air)ret-ton = return air (ton) before return fans
TAR to VAV = TBLD-AR + delta T due to return fans kW
VAVret-ton = return (ton) to VAV fans inlet
InfilVAV-Lat-ton = infiltration latent (ton) to VAV fans
VAVret-CFM = return CFM to VAV fans inlet
Kirby Nelson PE 2/9/2016
Exhaust air
ExLat-ton = latent load (ton) exhausted
ExCFM = CFM of exhaust air
TEx = temperature of exhaust air
Exsen-ton = sensible load (ton) exhausted
VAV Fans
Tret+FA = return and fresh air mix temperature (F)
(dh) = VAV air static pressure (ft)
Efan-VSD = VAV fans efficiency
VAVinlet-sen-ton = sensible load (ton) inlet to VAV fans
VAVinlet-lat-ton = latent load (ton) inlet to VAV fans
Tair-VAV = temp air to coils after VAV fan heat
(FAN)VAV-CFM = CFM air thru coils
(FAN)ton-VAV = load (ton) due to VAV fan kW
(FAN)kW-VAV = total VAV fan kW demand
AIR SIDE SYSTEM PLUS BUILDING
FAN kW = total air handlers kW
SITE kW = total site or air side kW
Plantton = load (ton) to plant
CENTRAL PLANT
Nomenclature will be defined by addressing each
component of the plant.
Primary/secondary pumping nomenclature
gpmevap = total gpm flow thru evaporators
(H)pri-total = total primary pump head (ft) = (H)pri-pipe +
(H)pri-fittings + (H)pri-bp + (H)evap
(H)pri-pipe = primary pump head due to piping (ft)
(H)pri-fittings = primary head due to pump & fitting (ft)
(Ef)c-pump = efficiency of chiller pump
Pc-heat-ton = chiller pump heat to atmosphere (ton)
Pc-kW = one chiller pump kW demand (kW)
Pchiller-# = number chiller pumps operating
(lwt)evap = temperature water leaving evaporator (F)
Tbp = temperature of water in bypass (F)
gpmbp = gpm water flow in bypass
(H)pri-bp = head if chiller pump flow in bypass (ft)
(ewt)evap = temp water entering evaporator (F)
Psec-heat-ton = secondary pump heat to atmosphere
(ton)
Psec-kW = kW demand of secondary pumps
Efdes-sec-p = design efficiency of secondary pumping
Efsec-pump = efficiency of secondary pumping
(H)sec = secondary pump head (ft) = (H)sec-pipe +
(H)sec-bp + (H)coil + (H)valve
(H)sec-pipe = secondary pump head due to pipe (ft)
(H)sec-bp = head in bypass if gpmsec > gpmevap
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gpmsec = water gpm flow in secondary loop
(ewt)coil = water temperature entering coil (F)
Plantton = load (ton) from air side to plant
Pipesize-in = secondary pipe size (inches)
(lwt)coil = temperature of water leaving coil (F)
Evaporator
(evap)ton = load (ton) on one evaporator
TER = evaporator refrigerant temp (F)
TER-app = evaporator refrigerant approach (F)
EVAPton = total evaporator loads (ton)
(H)evap = pump head thru evaporator (ft)
(evap)ft/sec = velocity water flow thru evaporator
(evap)des-ft/sec = evaporator design flow velocity
Compressor:
(chiller)kW = each chiller kW demand
(chiller)lift = (TCR – TER) = chiller lift (F)
(chiller)% = percent chiller motor is loaded
(chiller)# = number chillers operating
(CHILLER)kW = total plant chiller kW
(chiller)kW/ton = chiller kW per evaporator ton
Plant kW = total kW demand of plant
Condenser nomenclature:
(cond)ton = load (ton) on one condenser
TCR = temperature of condenser refrigerant (F)
TCR-app = refrigerant approach temperature (F)
(COND)ton = total load (ton) on all condensers
(H)cond = tower pump head thru condenser (ft)
(cond)ft/sec = tower water flow thru condenser
Tower piping nomenclature
Pipesize-in = tower pipe size (inches)
gpmT = each tower water flow (gpm)
(H)T-total = total tower pump head (ft)
PT-heat = pump heat to atmosphere (ton)
PT-kW = each tower pump kW demand
EfT-pump = tower pump efficiency
Ptower # = number of tower pumps
(H)T-pipe = total tower pump head (ft)
(ewt)T = tower entering water temperature (F)
(H)T-static = tower height static head (ft)
Trange = tower range (F)= (ewt)T – (lwt)T
(lwt)T = tower leaving water temperature (F)
Tapproach = (lwt)T – (Twet-bulb)
Tower nomenclature
tfan-kW = kW demand of one tower fan
Tfan-kW = tower fan kW of fans on
tfan-% = percent tower fan speed
Kirby Nelson PE 2/9/2016
tton-ex = ton exhaust by one tower
T# = number of towers on
Tton-ex = ton exhaust by all towers on
Trg+app = tower range + approach (F)
One hour performance indices
BLDkW = kW demand of building lights & plug loads
FankW = air side fans kW, VAV, return terminals
Ductheat = perimeter heat to air supply
FAheat = heat added to fresh air
Heattotal = total heat added to air
PlantkW = total plant kW
SystkW = total system kW
CCWSkW = air side + plant kW
ChillerkW/evap ton = chiller kW/evaporator ton
performance
PlantkW/site ton = plant kW per site or air side ton
CCWSkW/site ton = CCWS kW per load to plant
WeatherEin-ton = weather energy into the system
SitekW-Ein-ton = load (ton) due to site kW
PlantkW-Ein-ton = load (ton) due to plant kW
TotalEin-ton = total energy in to system (ton
Pumptot-heat-ton = total pump heat out (ton)
AHU Exlat ton = air exhausted latent ton
AHU Exsen ton = air exhausted sensible ton
Tower Tton Ex = energy exhausted by tower (ton)
Total Eout ton = total energy out of system (ton)
24 hour performance indices
BLD24hr-kW = building 24 hour kW usage
Fan24hr-kW = fan system 24 hour kW usage
Duct24hr-heat kW or therm = duct heat
FA24hr heat kW or therm = fresh air heat
Heat24hr total kW or therm = total heat into system air
Plant24hr kW = plant 24 hour kW usage
Syst24hr kW & therm = total system 24 hour energy
usage
(CCWS)24hr-kW = Central chilled water system (air
side + plant) 24 hour kW usage
Weather24hr-Ein-ton = 24 hour weather energy into
system
SITE24hr-kW-Ein-ton = 24 hour energy into site, building
& air side system
Plant24hr-kW-Ein-ton = 24 hour kW energy into plant
Total24hr-Ein-ton = total 24 hour energy into system
Pump24hr Heat out-ton = pump heat to atmosphere
(ton)
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AHU Ex24hr Lat ton = exhausted latent load from
building
AHU Ex24hr-sen-ton = exhausted sensible load from
building
Tower24hr out-ton = tower exhaust from system (ton)
Total E24hr-out-ton = total 24 hour energy out of syste
Kirby Nelson PE 2/9/2016
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Kirby Nelson PE 2/9/2016
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