EXHAUST AIR ENERGY
RECOVERY
ANALYSIS by SYSTEM ENERGY
EQUILIBRIUM (SEE) MODEL
The objective of a System Energy
Equilibrium (SEE) building energy model is
to duplicate the hourly performance of a
real building energy system 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 air side system, 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
investigate the energy savings potential of
an exhaust air energy recovery system
applied to a large office building located in
weather zone 4.
(SEE) MODEL CHARACTERISTICS
Understanding the performance of a
complex system, in this case a building and
(CCWS) that serves the building, requires a
model that includes detail model equations
of all components of the system. These
equations of each system component are
solved simultaneous, by computer, giving
the effect of each component on the
operation of the total system and the effect
of the system on the performance of the
component. Real building energy systems
operate according to the laws of
thermodynamics and the performance
characteristics of the equipment installed;
therefore the model must incorporate
equations that duplicate the laws of
thermodynamics and input the
characteristics of the system components
consistent with the manufactures verified
data. To accomplish this objective the
model must incorporate every design and
control feature of the real system; resulting
in a model, as presented here, consisting of
more than 150 performances and design
variables, each variable defined by an
equation and/or is a design constant that
changes if the design is changed. The set of
equations is solved simultaneously by
computer and will duplicate the
performance of a real system if sufficient
detail has been incorporated into the model
and the detail is consistent with the actual
equipment and controls of the real system.
The model is always at System Energy
Equilibrium (SEE).
The challenge developing a (SEE) model
might be summarized as; a real system is
very complex where minor changes in
weather, design, and control, can have a
major effect on the performance of the
system; therefore the system (SEE) model
must be equally complex incorporating all
characteristics of the real system within a
set of simultaneous equations solved by a
computer.
THE BUILDING & ASSUMED WEATHER
Figure 1 illustrates the building and Figure 2
the assumed weather for this analysis of
exhaust air energy recovery.
http://www.energycodes.gov/achieving30-goal-energy-and-cost-savings-analysisashrae-standard-901-2010. The building
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 2011).
Figure 1 Building
The building of this study is defined by the
Pacific Northwest National Laboratory (PNNL)
study of ASHRAE Standard 90.1-2010, a large 13
story office building, Figure 1, with 498,600
square feet of air conditioned space. The
Figure 2 Assumed peak summer design
weather
(PNNL) study is given by Liu, B. May 2011.
BUILDING SCHEMATIC
“Achieving the 30% Goal: Energy and Cost
Savings Analysis of ASHRAE Standard 90.12010” Pacific Northwest National
Laboratory.
Figure 3 defines the components of the
building schematic and nomenclature is
given at the end of this paper.
Figure 3 Building schematic defined
BLD ft2 = 498600
%clear sky = 100.0%
# floors = 13
Tdry-bulb = 99.8
2
Roof ft = 38,354
N/S wall ft2 = 40,560
InfilLat-ton = 30.84
Ex-/Infil+-CFM = 6811 <<
Twet-bulb= 77.2
Infilsen-ton = 15.2
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
GlassS trans ton =
GlassE-trans ton =
17.29
11.51
RoofTrans ton = 33.3
GlassW-trans ton =
11.51
Roofsky lite ton = 0.0
Peopleton-sen&lat = 59.5
GlassN-solar-ton =
7.1
plugton&kW = 93
Lightton&kW= 115
39.7
GlassS-solar-ton =
20.8
327.6
GlassE-solar ton =
403.9
GlassW-solar ton =
4.7
33.1
BLD kW=
731.4
Total Bldint-ton = 300.8
(int-cfm)to-per-ret= 176985
Tstat-int= 75.0
(Bld)int-air-ton= -300.8
^
479.1
SITE kW =
1210.5
Design
Tair supply int= 56.11
kW
17.7
62.4
GlassTot-solar-ton = 65.7
(int cfm)per-ton = 0.00 >
Tot Bldper-sen-ton = 156.1
v
Tstat-per = 75.0 return
^ (Bld)per-air-ton= -156.1
air
Tair supply per= 57.04
Design
ABS Bld Ton =
Ton
17.6
GlassTot-trans-ton= 57.6
4PM
ASHRAE
^
(fan)int-ter ton&kW=
FAN kW=
WallTot trans ton =
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
Interior
Tair coils = 55.00
duct
(D)int-CFM= 176,985
>>>(Coil)sen-ton= 682
(coil)cap-ton=
35.7
(coil)H2O-ft/sec=
1.10
(coil)des-ft/sec=
1.20
LMTD=
14.17
(COIL)L+s-ton=
855
<<<<
-173.8
Tair coils= 55.00
duct
(D)per-CFM= 96,565
(coil)gpm=
39.9
UAdesign=
COIL
^
^
2.52
(one coil)ton=
32.88
^
(H)coil=
1.8
(H)coil-des=
2.1
42
280,361 Return
(FAN)ret-kW=
81.8
Fan
(FAN)kW-VAV= 272.5
(FAN)ret-ton=
23.2
V
(Air)ret-ton =
527.9
^
26 VAV FANS
TFA to VAV =
99.8
> Tret+FA = 79.57
168.6
(dh) = 5.568
> (FA)CFM=
(FA)kW=
41,817
114.1
0.0
TBLD-AR = 75.00
(FAN)ton-VAV= 77.5
>(FA)sen-ton = >
> (FA)Lat-ton=
V
(Air)ret-CFM =
^
26 F.A.Inlet
^
^
2.66
UA=
Tair VAV= 82.72
(FAN)VAV-CFM= 273,551
statFA=
0.0
62.4
Peri
(D)per-air-ton=
^
^
0.0
Tar-to-VAV = 75.92
VAVret-sen ton = 436.3
VAVret Lat-ton = 58.28
< VAVret-CFM = 231,733 <
> Efan-VSD= 0.657
VAV inlet-sen-ton = 604.9
VAVinlet-lat-ton= 172.4
V
ExLat-ton = -12.2
ExCFM = -48,628
SEE SCHEMATIC air side
Air temp green kW red
Air CFM purple
Ton blue
TEx = 75.92
Exsen-ton = -91.6
V
v
Figure 4 Air side system-no exhaust energy
recovery at peak design weather
Figure 5 Air side system with exhaust
energy recovery at peak design weather
Figure 4 illustrates the air side system
without exhaust energy recovery and Figure
5 gives the effect of recovery. Note that all
system values are the same for the building,
duct system, and VAV kW fan performance.
With recovery, Figure 5, the exhaust air
temperature increases from 75.92F to
92.27F and the fresh air into the VAV fans
decreases from 99.8F, the outside dry bulb
temperature, to 80.8F; resulting in a drop of
load to the plant of (855 - 783 = 72 ton).
Exhaust air energy recovery only reduces
the load on the plant.
Figures 6 & 7 are of the VAV fan systems of
Figures 4 & 5.
Figure 6 VAV fan system of Figure 5-exhaust air recovery on
Figure 7 VAV fan system of Figure 4-no exhaust air recovery
Comparing Figures 6 & 7 illustrates the effect of exhaust air energy recovery at peak summer
design conditions. Transferring energy from the exhaust air to the fresh air decreases the air
into the VAV fans from 79.57F to 76.67F; resulting in a return air load to the VAV fans of 533.4
ton sensible load verses 604.9 ton without recovery. Note that the VAV kW and CFM are not
changed by the exhaust air recovery systems.
The load presented to the plant is reduced from 855 ton to 783 ton; let’s look at the response
of the plants.
2
BLD ft = 498600
Condenser
%clear sky = 100.0%
(cond)ton= 464
Pipesize-in = 6"
(H)T-pipe= 13.5
2
Tower
Roof ft = 38,354
Ex-/Infil+ CFM = 6811 <<
Twet-bulb= 77.2
Ex/Infilsen-ton = 15.2
TCR-app= 1.53
(H)T-total= 68.7
(H)T-static = 12.2
Tfan-kW=
24.8
N/S wall ft2 = 40,560
E/W wall ft2 = 27,008
(COND)ton= 928
PT-heat ton = -1.36
Trange= 12.4
tfan-% =
100%
Wall % glass= 37.5%
(H)cond= 43.0
< pT-kW= 28.1
< (lwt)T = 85.0
tton-ex=
-467
Glass U = 0.55
WallWtranston=
3.28
EfTpump= 0.83
Tapproach = 7.8
T #=
2
Wall U = 0.09
GlassN trans ton =
17.29
T-Ton-ex=
Trg+app =
20.2
Glass SHGC = 0.40
Wall emitt = 0.55
GlassS trans ton =
GlassE-trans ton =
17.29
11.51
RoofTrans ton = 33.3
GlassW-trans ton =
11.51
Roofsky lite ton = 0.0
Peopleton sen&lat = 59.5
GlassN-solar-ton =
7.1
20.8
TCR= 98.9
(cond)ft/sec= 9.7
> gpmT= 1800
> (ewt)T= 97
tfan-kW=
Ptower # = 2
Compressor
12.4
-935
ASHRAE Design
(chiller)kW= 226.5
(chiller)lift= 56.9
St Louis
Large
90.1-2010 #people
Office
2380
(chiller)% = 89%
(chiller)#= 2
Peak day
Weather
Design
%clear sky =
4PM
1.00
plugton&kW = 93
Lightton&kW= 115
conditions
Tdry bulb =
Twet bulb =
99.8
77.2
Total Bldint-ton = 300.8
(int-cfm)to-per-ret= 176985
(CHILLER)kW= 453.0
(chiller)kW/ton= 0.572
Plant kW =
39.7
(fan)int-ter ton&kW=
WallStrans ton=
5.30
WallEtrans ton=
4.09
GlassE-solar ton =
403.9
GlassW-solar ton =
4.7
33.1
BLD kW=
FAN kW=
731.4
479.1
SITE kW =
^
kW
17.7
62.4
GlassTot-solar-ton = 65.7
(int cfm)per-ton = 0.00 >
Tot Bldper-sen-ton = 156.1
v
Tstat-per = 75.0 return
^ (Bld)per-air-ton= -156.1
456.91
^
(fan)per-ter ton&kW=
TER= 42.0
Ton
kW
17.7
62.4
0.0
0.0
Treheat air = 55.0
(H)evap= 34.5
(D)reheat ton&kW =
0.0
0.0
(evap)ft/sec= 8.38
62.4
(evap)des-ft/sec= 8.38
(D)int-air-ton= -318.6
Interior
V
Tair coils = 55.00
duct
gpmevap= 1200
Psec-heat-ton =
-2.1
> Psec-kW= 28.6
(lwt)evap = 44.43
(H)pri-total= 44.1
^
(D)int-CFM= 176,985
> (ewt)coil= 44.4
>>>(Coil)sen-ton=
611
v
Efdes-sec-p = 0.80
(coil)cap-ton=
30.7
(H)pri-pipe= 2.5
Tbp=
44.43
Efsec-pump = 0.75
(coil)H2O-ft/sec=
1.00
(H)pri-fitings= 7.0
gpmbp=
-249
PLANTton = 783
(coil)des-ft/sec=
1.20
(Ef)c-pump= 0.81
(H)pri-bp=
0.11
LMTD=
12.83
(H)sec-bp= 0.00
Pipesize-in = 8.0
(COIL)L+s-ton=
< (gpm)sec= 951
< (lwt)coil= 64.4
Pc-heat-ton= -0.66
^
< pc-kW= 12.3
v
(ewt)evap =
60.27
(H)sec= 119
(H)sec-pipe=
58
783
<<<<
chillerkW/evapton= 0.572
4PM
(plant)kW/site ton= 0.698
Design
Peoplesen+lat ton =
99.2
All Electric
kW
BLD.kW=
731.4
(Fan)kW =
479.1
WeatherEin-ton = 431.4
Ductheat= 0.0
(Site)kW-Ein-ton = 344.3
(FA)heat= 0.0
PlantkW-Ein-ton = 155.5
Heat total =
Total Ein-ton = 1030
PlantkW=
546.9
SystkW =
1757.4
BLD.kW=
731.4
Pumptot-heat-ton =
-4.1
Fuel Heat
THERM
0.00
Plant
0.0
Tower Tton-Ex =
Einternal energy chg =
SEE
SCHEMATIC
Ton
Blue
kW
Red
-935 CCWSkW =
1025.9
Water temp pink
12.3
1757.4
Water gpm orange
Total Eout-ton = -1030
SystkW =
St Louis
air
temp green
Peri
duct
(D)per-CFM= 96,565
^
(coil)gpm=
36.6
UAdesign=
2.66
COIL
^
^
2.39
(one coil)ton=
30.12
(H)coil=
1.5
(H)coil-des=
2.1
280,361 Return
81.8
Fan
(FAN)kW-VAV= 272.5
(FAN)ret-ton=
23.2
V
(Air)ret-ton =
527.9
^
^
26 VAV FANS
80.8
> Tret+FA = 76.67
97.1
(dh) = 5.568
(FA)kW=
41,817
0.0
TBLD-AR = 75.00
(FAN)ret-kW=
42
114.1
V
(FAN)ton-VAV= 77.5
TFA to VAV =
> (FA)CFM=
^
(Air)ret-CFM =
>(FA)sen-ton = >
> (FA)Lat-ton=
^
UA=
^
26 F.A.Inlet
statFA=
0
0.0
-173.8
Tair coils= 55.00
^
Exhaust air recovery ON
AHU ExLat-ton = -12.2
AHU Exsen-ton = -91.6
Tdry bulb = 99.8
0.00
(D)per-air-ton=
Tair VAV= 79.81
(FAN)VAV-CFM= 273,551
Pchiller-# = 2
CCWSkW/bld ton= 2.25
V
Theat-air= 55.0
(D)heat ton&kW =
EVAPton= 792
^
air
Tair supply per= 57.04
Design
TER-app= 2.39
^
17.6
GlassTot-trans-ton= 57.6
4PM
ABS Bld Ton =
Ton
WallTot trans ton =
1210.5
Design
ASHRAE
^
(evap)ton= 396.1
4.92
GlassS-solar-ton =
Tair supply int= 56.11
Evaporator
WallNtrans ton=
327.6
Tstat-int= 75.0
546.9
(Bld)int-air-ton= -300.8
>
Ex/InfilLat-ton = 30.84
Tdry-bulb = 99.8
# floors = 13
Tar-to-VAV = 75.92
VAVret-sen ton = 436.3
VAVret Lat-ton = 58.28
< VAVret-CFM = 231,733 <
> Efan-VSD= 0.657
VAV inlet-sen-ton = 533.4
VAVinlet-lat-ton= 172.4
V
ExLat-ton = -12.2
ExCFM = -48,628
SEE SCHEMATIC air side
Air temp green kW red
Air CFM purple
Ton blue
TEx = 75.92
Exsen-ton = -91.6
Exair recovery
ON
Tex-recoc = 92.27
V
v
Figure 8 Total system with exhaust air energy recovery.
Comparing 8 & 9 illustrates the plant kW reduces from 597.4 kW to 546.9 kW and the total system kW
reduces from 1807.9 kW to 1757.4 kW at peak summer design conditions. Each chiller kW reduces
from 249.4 kW to 226.5 kW. A 9% smaller chiller with exhaust air energy recovery; the only
significant effect of exhaust air energy recovery as we will show with an analysis of 24 hour
energy consumption.
Figure 9 Total system without exhaust air energy recovery
Figure 10: 24 hour performance with
recovery
Figure 10 illustrates the 24 hour
performance of the system with exhaust
air energy recovery. The bottom chart
illustrates the maximum possible energy
transfer to the fresh air as a function of
air CFM and delta temperature. For the
conditions of this design and the fact
that it is a one shift office building; the
exhaust air has more max possible
energy than the fresh air. The bottom
chart also gives the energy transferred
to the fresh air.
The top chart of Figure 10 shows the
fresh air CFM drops off and therefore
the tons transferred to the fresh air
drops off. The top chart also shows the
air temperatures illustrating the
difference in the dry bulb temperature
of outside air and the approximate
75.8F exhaust air drives the amount of
energy available for transfer to the fresh
air.
A building that operates more than one
shift would have greater 24 hour
transfer of energy to the fresh air.
Figure 11, 12, & 13 illustrates the
exhaust air energy recovery has
moderate effect on 24 hour energy
consumption at peak design summer
weather conditions.
Figure 11: 24 hour performance
BLD sq-ft = 498,600
ALL ELECTRIC Peak day
Design
24hr
BLD.24hr-kW=
10,096
(Fan)24hr-kW =
6,482
(Duct)24hr-heat kW= 0
(FA)24hr-heat kW= 0
Heat24hr-total kW=
0
Plant24hr-kW=
7,688
SYST 24hr-kW =
24,267
(CCWS)24hr-kW=
14,170
BLD.24hr-kW=
10,096
Total24hr-kW =
24,267
People24hr Ein ton =
950.0
Weather24h-Ein-ton=
6059.6
SITE24h-kW-Ein-ton =
4715.2
Plant24h-kW-Ein-ton =
2186.6
Total24h-Ein-ton =
13911.4
Pump24hr-heat-ton =
-79.3
AHU Ex24hr-Lat-ton = -181.5
AHU Ex24hr-sen-ton = -1247.9
Tower24hr-ton-Ex = -12696.4
E24hr chg internal energy = 294.23
Total E24hr-out-ton = -13910.9
ASHRAE
Figure 12: 24 hour performance
without exhaust air energy recovery
Figure 13: 24 hour performance with
exhaust air energy recovery
CONCLUSION FOR PEAK DESIGN DAY
WEATHER CONDITIONS
Exhaust air energy recovery reduces the
chiller kW about 9% for the conditions
studied here.
Next we will consider average summer
conditions.
TYPICAL SUMMER WEATHER
Figure 14: Summer weather
Figure 16 System 24 hour performance
with exhaust air recovery
CONCLUSION
Figure 15 24 hour system kW demand
Figure 14 gives the assumed typical
summer weather and Figure 15 gives
the 24 hour system kW for the system
with and without exhaust air energy
recovery illustrating very little effect
with exhaust air energy recovery.
For the conditions studied here exhaust
air energy recovery has be shown to
have very little effect on the total
energy consumption of the system. This
study showed that the kW size of the
chiller is reduced about 9% if exhaust air
energy recovery is installed in all 26 VAV
fan systems.
ASHRAE Standard 90.1-2013
requirement for exhaust air energy
recovery is at best questionable.
NOMENCLATURE (NOTE see other
papers at this site for additional
understanding of the system and
nomenclature)
Air Side System Nomenclature
Each of the more than 100 variables of
the air side system will be 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 sen&lat = sensible & latent
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)
(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)
Ex/Infillat-ton = latent load due to air
infiltration or exfiltration (ton)
Ex/InfilCFM = air infiltration or exfiltration
CFM
Exfilsen-ton =sensible load due to air
exfiltration or infiltration (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
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)
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) transferred to Plant
(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-sen ton = return sensible (ton) to
VAV fans inlet
VAVret-lat ton = return latent (ton) to VAV
fans inlet
VAVret-CFM = return CFM to VAV fans inlet
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 (in)
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 = (COIL)L+s ton 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
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
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
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
Tower24hr out-ton = tower exhaust from
system (ton)
Total E24hr-out-ton = total 24 hour energy out
of system
Total24hr-Ein-ton = total 24 hour energy
into system
Pump24hr Heat out-ton = pump heat to
atmosphere (ton)
AHU Ex24hr Lat ton = exhausted latent load
from building
AHU Ex24hr-sen-ton = exhausted sensible
load from building