RESETTING VAV DUCT STATIC PRESSURE SET POINTSteve 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) MODELINGThe 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 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/LIMITATIONSThe 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

Kirby Nelson PE 5/5/2023 Page 1

FIG. 2 Weather conditions for assumed 24 hours peak design day

AIR SIDE SCHEMATICFigure 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.

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).

Kirby Nelson PE 5/5/2023 Page 2

BLD ft2 = 498600 %clear sky = 100.0% InfilLat-ton = 30.84# floors = 13 Tdry-bulb = 99.8 Infil-CFM = 6811 <Roof ft2 = 38,354 Twet-bulb= 77.2 Infilsen-ton = 15.2

N/S wall ft2 = 40,560 WallNtrans ton= 4.92E/W wall ft2 = 27,008 WallStrans ton= 5.30

Wall % glass= 37.5% WallEtrans ton= 4.09Glass U = 0.55 WallWtranston= 3.28 WallTot trans ton = 17.6

Wall U = 0.09 GlassN trans ton = 17.29Glass SHGC = 0.40 GlassS trans ton = 17.29

Wall emitt = 0.55 GlassE-trans ton = 11.51RoofTrans ton = 33.3 GlassW-trans ton = 11.51 GlassTot-trans-ton= 57.6Roofsky lite ton = 0.0 GlassN-solar-ton = 7.1

Peopleton = 60 kW GlassS-solar-ton = 20.8plugton&kW = 93 328 GlassE-solar ton = 4.7Lightton&kW= 115 404 GlassW-solar ton = 33.1 GlassTot-solar-ton = 65.7

Total Bld int-ton = 300.8 BLD kW= 731.4 (int cfm)per-ton = 0.00 >(int-cfm)to-per-ret= 176985 FAN kW= 479.1 Tot Bldper-sen-ton = 156.1 v

Tstat-int= 75.0 SITE kW = 1210.5 Tstat-per = 75.0 return(Bld)int-air-ton= -300.8 ^ Design 4PM ^ (Bld)per-air-ton= -156.1 air

Tair supply int= 56.11 ASHRAE Design Tair supply per= 57.04 ^ ABS Bld Ton = 456.91 ^

Ton kW Ton kW V(fan)int-ter ton&kW= 17.7 62.4 (fan)per-ter ton&kW= 17.7 62.4

Theat-air= 55.0 (D)heat ton&kW = 0.0 0.0

Treheat air = 55.0(D)reheat ton&kW = 0.0 0.0

62.4(D)int-air-ton= -318.6 Interior (D)per-air-ton= -173.8 Peri

Tair coils = 55.00 duct Tair coils= 55.00 duct(D)int-CFM= 176,985 ^ (D)per-CFM= 96,565 ^

>>>(Coil)sen-ton= 682 ^ (coil)gpm= 38.4 ^(coil)cap-ton= 35.7 UAdesign= 2.66

(coil)H2O-ft/sec= 1.05 COIL UA= 2.46(coil)des-ft/sec= 1.20 (one coil)ton= 31.62

LMTD= 14.52 (H)coil= 1.6 V(COIL)L+s-ton= 822 ^ ^ ^ (H)coil-des= 2.1

<<<< Tair VAV= 82.72 TBLD-AR = 75.00(FAN)VAV-CFM= 273,551 (Air)ret-CFM = 280,361 Return(FAN)ton-VAV= 77.5 (FAN)ret-kW= 82 Fan(FAN)kW-VAV= 273 (FAN)ret-ton= 23.2 V

^ (Air)ret-ton = 527.926 F.A.Inlet ^ Tar-to-VAV = 75.92

statFA= 42 26 VAV FANS VAVret-ton = 436.3 TFA to VAV = 99.8 > Tret+FA = 79.57 InfilVAV-Lat-ton = 25.49

>(FA)sen-ton = > 168.6 (dh) = 5.568 < VAVret-CFM = 231,733 <> (FA)CFM= 41,817 > Efan-VSD= 0.657 V

> (FA)Lat-ton= 114.1 VAV inlet-sen-ton = 604.9(FA)kW= 0.0 VAV inlet-lat-ton= 139.6 ExLat-ton = -5.3

ExCFM = -48,628

SEE SCHEMATIC air side TEx = 75.92Air temp green kW red Exsen-ton = -91.6 V Air CFM purple Ton blue v

FIG. 3 Air Side Schematic at peak conditions. VAV fan kW = 273. (dh)=5.568in

Kirby Nelson PE 5/5/2023 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.

40,752

159,514 176,985

61,36373,970

96,565

30,832

59.8 60.3 60.3 59.9

57.056.2 56.2 56.2 56.1

56.957.6 58.2

61.962.7 63.3

61.0

58.657.7 57.3 57.0

61.4

52

54

56

58

60

62

64

66

68

70

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

FAN VAV-CFM

AIR

TEM

P. (F

)

(CFM

)

FAN kW-VAV(ASHRAE Design)SUPPLY AIR TEMP-CFM & VAV kW-CFM

(Duct)interior-CFM (Duct)perimeter-CFM(Temp)air-supply-interior (Temp)air-supply-perimeter

FIG. 4 VAV fan CFM & air temp with duct pressure control

FIG. 4A VAV fan CFM & air temp with constant 5.5in duct pressure

FIG. 5 Fan system kW 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.

Kirby Nelson PE 5/5/2023 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.

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.

Kirby Nelson PE 5/5/2023 Page 5

FIG. 8 System kW with duct pressure control

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.

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.

Kirby Nelson PE 5/5/2023 Page 6

BLD sq-ft = 498,600ALL ELECTRIC Peak day

Design 24hr BLD.24hr-kW= 10,096

(Fan)24hr-kW = 8,263(Duct)24hr-heat kW= 0

(FA)24hr-heat kW= 0Heat24hr-total kW= 0

Plant24hr-kW= 7,683SYST 24hr-kW = 26,043

(CCWS)24hr-kW= 15,946BLD.24hr-kW= 10,096

Total24hr-kW = 26,043Weather24h-Ein-ton= 6923SITE24h-kW-Ein-ton = 5222Plant24h-kW-Ein-ton = 2185Total24h-Ein-ton = 14330Pump24hr-heat-ton = -80

AHU Ex24hr-Lat-ton = -110AHU Ex24hr-sen-ton = -1265

Tower24hr-ton-Ex = -12875Total E24hr-out-ton = -14330

ASHRAE Design

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

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 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?

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

Kirby Nelson PE 5/5/2023 Page 7

BLD ft2 = 498600 %clear sky = 100.0% InfilLat-ton = 27.50# floors = 13 Tdry-bulb = 87.0 infil-CFM = 6811 <<Roof ft2 = 38,354 Twet-bulb= 73.0 Infilsen-ton = 7.4

N/S wall ft2 = 40,560 WallNtrans ton= 2.28E/W wall ft2 = 27,008 WallStrans ton= 2.28

Wall % glass= 37.5% WallEtrans ton= 2.00Glass U = 0.55 WallWtranston= 1.52 WallTot trans ton = 8.1

Wall U = 0.09 GlassN trans ton = 8.37Glass SHGC = 0.40 GlassS trans ton = 8.37

Wall emitt = 0.55 GlassE-trans ton = 5.57RoofTrans ton = 1.8 GlassW-trans ton = 5.57 GlassTot-trans-ton= 27.9Roofsky lite ton = 0.0 GlassN-solar-ton = 6.1

Peopleton = 60 kW GlassS-solar-ton = 6.6plugton&kW = 93 328 GlassE-solar ton = 55.4Lightton&kW= 115 404 GlassW-solar ton = 4.1 GlassTot-solar-ton = 72.1

Total Bld int-ton = 269.4 BLD kW= 731.4 (int cfm)per-ton = 0.00 >(int-cfm)to-per-ret= 159514 FAN kW= 372.6 Tot Bldper-sen-ton = 115.4 v

Tstat-int= 75.0 SITE kW = 1104.0 Tstat-per = 75.0 return(Bld)int-air-ton= -269.4 ^ Design 10AM ^ (Bld)per-air-ton= -115.4 air

Tair supply int= 56.24 ASHRAE Design Tair supply per= 57.67 ^ ABS Bld Ton = 384.79 ^

Ton kW Ton kW V(fan)int-ter ton&kW= 17.7 62.4 fanper-ter ton&kW= 17.7 62.4

Theat-air= 55.0 (D)heat ton&kW = 0.0 0.0

Treheat air = 55.0(D)reheat ton&kW = 0.0 0.0

62.4(D)int-air-ton= -287.1 Interior (D)per-air-ton= -133.1 Peri

Tair coils = 55.00 duct Tair coils= 55.00 duct(D)int-CFM= 159,514 ^ (D)per-CFM= 73,970 ^

>>>(Coil)sen-ton= 533 ^ (coil)gpm= 30.2 ^(coil)cap-ton= 28.6 UAdesign= 2.66

(coil)H2O-ft/sec= 0.83 COIL UA= 2.13(coil)des-ft/sec= 1.20 (one coil)ton= 24.93

LMTD= 13.43 (H)coil= 1.0 V(COIL)L+s-ton= 648 ^ ^ ^ (H)coil-des= 2.1

<<<< Tair VAV= 80.35 TBLD-AR = 75.00(FAN)VAV-CFM= 233,484 (Air)ret-CFM = 240,295 Return(FAN)ton-VAV= 54.2 (FAN)ret-kW= 57 Fan(FAN)kW-VAV= 191 (FAN)ret-ton= 16.3 V

^ (Air)ret-ton = 448.826 F.A.Inlet ^ Tar-to-VAV = 75.75

statFA= 42 26 VAV FANS VAVret-ton = 358.0 TFA to VAV = 87.0 > Tret+FA = 77.77 InfilVAV-Lat-ton = 21.93

>(FA)sen-ton = > 120.4 (dh) = 4.432 < VAVret-CFM = 191,667 <> (FA)CFM= 41,817 > Efan-VSD= 0.638 V

> (FA)Lat-ton= 93.6 VAV inlet-sen-ton= 478.4(FA)kW= 0.0 VAV inlet-lat-ton= 115.5 ExLat-ton = -5.6

ExCFM = -48,628

SEE SCHEMATIC air side TEx = 75.75Air temp green kW red Exsen-ton = -90.8 V Air CFM purple Ton blue v

Kirby Nelson PE 5/5/2023 Page 8

Condenser(cond)ton= 382 Pipesize-in = 6" (H)T-pipe= 13.5 Tower

TCR= 92.2 > gpmT= 1800 > (ewt)T= 90.8 tfan-kW= 8.3TCR-app= 1.42 (H)T-total= 66.4 (H)T-static = 9.9 Tfan-kW= 16.6

(COND)ton= 765 PT-heat = -1.31 Trange= 10.2 tfan-% = 100%(H)cond= 43.0 < pT-kW= 27.1 < (lwt)T = 80.6 tton-ex= -385

(cond)ft/sec= 9.7 EfTpump= 0.83 Tapproach = 7.6 T#= 2Ptower # = 2 T-Ton-ex= -769

Trg+app = 17.8Compressor ASHRAE Design

(chiller)kW= 181 St Louis 90.1-2010(chiller)lift= 50.1 Large Office(chiller)% = 73% Peak day Design 10AM(chiller)#= 2 Weather %clear sky = 100%

(CHILLER)kW= 363 conditions Tdry bulb = 87.0(chiller)kW/ton= 0.554 Twet bulb = 73.0Plant kW = 440.1

> Evaporator(evap)ton= 327.6

TER= 42.1TER-app= 1.95

^ EVAPton= 655(H)evap= 34.5

(evap)ft/sec= 8.38(evap)des-ft/sec= 8.38

^ Vgpmevap= 1200 Psec-heat-ton = -1.80

(lwt)evap = 44.07 > Psec-kW= 21.2 > (ewt)coil= 44.1(H)pri-total= 44.3 v Efdes-sec-p = 0.80

^ (H)pri-pipe= 2.5 Tbp= 44.07 Efsec-pump = 0.70(H)pri-fitings= 7.0 gpmbp= -414 (H)sec= 100.6 PLANT ton = 648(Ef)c-pump= 0.81 (H)pri-bp= 0.30 (H)sec-pipe= 40Pc-heat-ton= -0.67 v (H)sec-bp= 0.00 Pipesize-in = 8.0

^ < pc-kW= 12.4 (ewt)evap = 57.17 < (gpm)sec= 786 < (lwt)coil= 64.1Pchiller-# = 2

St Louis 10AM All Electric Fuel HeatPerformance 10AM Design kW THERMchillerkW/evapton= 0.554 BLD.kW= 731.4

plantkW/site ton= 0.679 (Fan)kW = 372.6CCWSkW/bld ton= 2.11 Ductheat= 0.0 0.00WeatherEin-ton = 430.5 (FA)heat= 0.0 0.00(Site)kW-Ein-ton = 314.0 Heat total = 0.0 0.00PlantkW-Ein-ton = 125.2 PlantkW= 440.1 Plant

Total Ein-ton = 870 SystkW = 1544.1 1544.1 SEE SCHEMATICPumptot-heat-ton = -3.8 Ton Blue

AHU ExLat-ton = -5.6 BLD.kW= 731.4 kW RedAHU Exsen-ton = -90.8 CCWSkW = 812.7 Water temp pinkTower Tton-Ex = -769 SystkW = 1544.1 Water gpm orangeTotal Eout-ton = -870 air temp green

Kirby Nelson PE 5/5/2023 Page 9

191,667

231,733

85,956

1,125 499 499 499

32,4

22

41,8

17

41,8

17

41,8

17

41,8

17

33,6

74

11,2

25

6,23

9

69,330 60,740

233,484273,551

92,19575.57 75.75 75.92

77.7680.35

82.72 78.04

60

65

70

75

80

85

90

95

020,00040,00060,00080,000

100,000120,000140,000160,000180,000200,000220,000240,000260,000280,000

174 157 151 174 392 648 695 755 822 590 372 241

26 Coils sensible ton

AIR

TEM

PERA

TURE

(F)

(CFM

)

Plant load = 26 Coils latent + sensible ton

(ASHRAE Design)CFM & AIR Temp TO COIL & COIL LOADS

(VAV)ret-CFM (FA)-CFM (FAN)VAV-CFM(Temp)air ret-to-VAV (Temp)air-VAV to coil

References1. 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-goal-energy-and-cost-savings-analysis-ashrae-standard-901-2010

2.Taylor, S. 2011. “Optimizing Design & Control of Chilled Water Plants.” ASHRAE Journal (12)

NOMENCLATUREEach of the more than 100 variables of the system defined.Building structure;BLD ft2 = air conditioned space# Floors = number of building floorsRoof ft2 = roof square feetN/S wall ft2 =north/south wall square feetE/W wall ft2 =east/west wall square feetWall % glass = percent of each wall that is glassGlass U = glass heat transfer coefficientWall U = wall heat transfer coefficientGlass SHGC = glass solar heat gain coefficientWall emit = wall solar indexBuilding 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 loadsLightton&kW = cooling load & kW due to lightsTotal Bldint-ton = total building interior load (ton)

(int-cfm) to-per-return = CFM of interior supply air that returns to perimeter of buildingTstat-int = interior stat set temperature (F)Bldint-air-ton = supply air ton to offset interior loadBLD kW = total building kW demandBuilding perimeter space;%clear sky = percent clear skyTdry 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 CFMExfilCFM = air exfiltration CFMInfilsen-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 systemInterior 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 ductTair coils = supply air temperature off coils to duct (F)(D)int-CFM = supply air CFM to building interior ductPerimeter ductTair supply per =temp (F) air supply to building perimeter (fan)per ter ton&kW = perimeter ton & kW of terminal fansTheat-air = temp supply air before terminal fan heat (F)

Kirby Nelson PE 5/5/2023 Page 10

(D)heat-ton&kW = heat to perimeter supply air ton & kWTreheat 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 coilUAdesign = coil UA design valueUA = 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 systemFresh airstatFA = 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 temperatureAir 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 fansTAR to VAV = TBLD-AR + delta T due to return fans kWVAVret-ton = return (ton) to VAV fans inletInfilVAV-Lat-ton = infiltration latent (ton) to VAV fansVAVret-CFM = return CFM to VAV fans inletExhaust air ExLat-ton = latent load (ton) exhaustedExCFM = CFM of exhaust airTEx = temperature of exhaust air

Exsen-ton = sensible load (ton) exhaustedVAV Fans Tret+FA = return and fresh air mix temperature (F)(dh) = VAV air static pressure (ft)Efan-VSD = VAV fans efficiencyVAVinlet-sen-ton = sensible load (ton) inlet to VAV fansVAVinlet-lat-ton = latent load (ton) inlet to VAV fansTair-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 demandAIR SIDE SYSTEM PLUS BUILDINGFAN kW = total air handlers kWSITE kW = total site or air side kWPlantton = load (ton) to plantCENTRAL PLANT Nomenclature will be defined by addressing each component of the plant.Primary/secondary pumping nomenclaturegpmevap = 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 pumpPc-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 pumpsEfdes-sec-p = design efficiency of secondary pumpingEfsec-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 plantPipesize-in = secondary pipe size (inches)

Kirby Nelson PE 5/5/2023 Page 11

(lwt)coil = temperature of water leaving coil (F)Evaporator(evap)ton = load (ton) on one evaporatorTER = 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 velocityCompressor:(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 tonPlant kW = total kW demand of plantCondenser nomenclature:(cond)ton = load (ton) on one condenserTCR = 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 condenserTower piping nomenclaturePipesize-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 demandEfT-pump = tower pump efficiencyPtower # = 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 fanTfan-kW = tower fan kW of fans ontfan-% = percent tower fan speedtton-ex = ton exhaust by one tower

T# = number of towers onTton-ex = ton exhaust by all towers onTrg+app = tower range + approach (F)

One hour performance indicesBLDkW = kW demand of building lights & plug loadsFankW = air side fans kW, VAV, return terminalsDuctheat = perimeter heat to air supplyFAheat = heat added to fresh airHeattotal = total heat added to airPlantkW = total plant kWSystkW = total system kWCCWSkW = air side + plant kWChillerkW/evap ton = chiller kW/evaporator ton performancePlantkW/site ton = plant kW per site or air side tonCCWSkW/site ton = CCWS kW per load to plantWeatherEin-ton = weather energy into the systemSitekW-Ein-ton = load (ton) due to site kWPlantkW-Ein-ton = load (ton) due to plant kWTotalEin-ton = total energy in to system (tonPumptot-heat-ton = total pump heat out (ton)AHU Exlat ton = air exhausted latent tonAHU Exsen ton = air exhausted sensible tonTower Tton Ex = energy exhausted by tower (ton)Total Eout ton = total energy out of system (ton)24 hour performance indicesBLD24hr-kW = building 24 hour kW usageFan24hr-kW = fan system 24 hour kW usageDuct24hr-heat kW or therm = duct heatFA24hr heat kW or therm = fresh air heatHeat24hr total kW or therm = total heat into system airPlant24hr kW = plant 24 hour kW usageSyst24hr kW & therm = total system 24 hour energy usage(CCWS)24hr-kW = Central chilled water system (air side + plant) 24 hour kW usageWeather24hr-Ein-ton = 24 hour weather energy into systemSITE24hr-kW-Ein-ton = 24 hour energy into site, building & air side systemPlant24hr-kW-Ein-ton = 24 hour kW energy into plantTotal24hr-Ein-ton = total 24 hour energy into systemPump24hr Heat out-ton = pump heat to atmosphere (ton)AHU Ex24hr Lat ton = exhausted latent load from buildingAHU Ex24hr-sen-ton = exhausted sensible load from buildingTower24hr out-ton = tower exhaust from system (ton)Total E24hr-out-ton = total 24 hour energy out of syste

Kirby Nelson PE 5/5/2023 Page 12

Kirby Nelson PE 5/5/2023 Page 13