SYSTEM ENERGY EQUILIBRIUM (SEE) CHILLER PLANT ... · Web view2012 ASHRAE HANDBOOK, HVAC Systems and...

System Energy Equilibrium (SEE) Chiller Plant ModelIntroductionMy career in building energy management and energy modeling began the fourth quarter of 1973 when the “Oil Embargo” occurred. I was a military systems design engineer with Texas Instruments Inc. (TI) and a member of a small group that simulated or modeled systems by computers for the purpose of design and evaluating the performance of the systems. The systems we modeled that I was personally involved with included the Shrike missile & Lacer guided bombs flight control, gas dynamics of pneumatic actuators, cannon internal explosion & projectile exit speed, and the dynamics of impact of electronics with soil and the dynamics of metal compression. This experience led me to the task, in 1974, of modeling the energy consumption of TI buildings for the purpose of defining what could be done to reduce energy consumption. I used building energy computer programs available at the time that were based on simulating the yearly energy use of a building based on utility bills. I became convinced that the need was for a building energy model that could model the last 24 hours of energy use and could also model the real time energy use of a building at any hour. To accomplish this a building energy model would require the same level of detail and sophistication as was required to model military systems. I spent years looking for this 24-hour building energy model, but it never showed. On retiring I decided to develop my own and here is the first of a series of papers that verify the (SEE) Plant Model. The (SEE) Plant Model can be reviewed at http://kirbype.com/ . Best regards to all. Kirby Nelson P.E.Life Member ASHRAE

(SEE) Plant Model ObjectivesThe objective of a System Energy Equilibrium (SEE) Plant Model is to duplicate the hourly performance of real plant at all operational conditions. To

accomplish this the (SEE)Model must obey the laws of thermodynamics and input the nonlinear characteristics of the real plant components and model all details of the plant that contribute to the performance of the real plant. The objective is to develop a math model, solved by computer, of the real plant that duplicates the real time performance of the real plant at all operating conditions within 3%.

Data Used to Verify (SEE) Chiller Plant Model

The development of the (SEE) Chiller Plant Model used four data sources to show the (SEE) Plant Model duplicates manufacture’s data. First Schwedler1 provides chiller/tower performance data for three plants as the load drops from 1,000-ton design load down to 300-ton load and the wet bulb drops about 12F degrees below design wet bulb. Design wet bulb is different for each of the three cities. Second the tower performance was verified against a manufacture’s computer program for tower selection and performance3. Third, a chiller manufacture provided chiller data that included the evaporator and condenser refrigerant approach temperatures showing how it affects chiller kW. Forth the ASHRAE Handbook6 gives water & refrigerant temperature relations in a centrifugal chiller.

Why Chiller Plant Model?

The first task of a chiller plant model is the design of the plant. The second task, after the plant is operational, is to answer the question, “is the plant operating as designed?” A difficult question to answer because the plant may never operate at peak design conditions and if it did would data be taken? A (SEE)Model of the plant offers the opportunity to visit the plant on any day under any conditions of weather and load and the model will define performance compared to “as designed performance” of the plant as it operates at part load conditions. Third if the plant is not operating as designed then the (SEE)Model provides a tool to evaluate and define why plant performance is substandard. Fourth the (SEE) Model provides a

http://kirbype.com/

means of defining control strategies or perhaps become part of the control software. Fifth the (SEE) Model offers the ability to visit the plant in future years and define plant degradation due to aging, bad control practice or other issues. Sixth the (SEE) Model offers a design tool for defining upgrading or expansion of the plant. And seventh a (SEE) model provides the opportunity to evaluate issues raised by plant operators. In summary a (SEE) plant model provides;

1. Plant design tool.2. Answers “Is the plant operating as

designed”?3. Help define why plant performance is

substandard.4. Define plant control strategies.5. Define causes of plant degradation/ageing.6. Define upgrading or expansion of plant.7. Model issues raised by plant operators

and/or technical publications.

This paper will show that the (SEE) Plant Model duplicates the chiller/tower data of Schwedler1, the chiller evaporator & condenser data provided by a chiller manufacture2, and tower selection data provided by Marley3. This paper will also show that the model mimics ASHRAE Handbook6 data that gives water and refrigerant temperature profiles in a centrifugal chiller. Model verification (agreement of model with test data) was an absolute necessary step in my career as a designer of military products and therefore a necessary step here.

The challenge for the (SEE) Plant Model is to duplicate the data of Schwedler1, Manufacture2 and Marley3 and do so with the same set of equations, changing only the design tower and/or chiller.

Building plus Plant (SEE)Model

The principles of modeling demonstrated in this series of papers on plant modeling is applied to the modeling of a large office building defined by Pacific Northwest National laboratory, Liu12. The building (SEE)Model11 can evaluate total building performance at real weather conditions13 and

therefore provide building benchmark data at real weather conditions for any day. Go to http://kirbynelsonpe.com/ to review.

(SEE) Plant Model Description.

The model consists of about 126 equations, 46 design inputs, and 8 controls.

ITEM Design input Equations CONTROLTower 13 26 Fan speedTower pump 6 13 GPMChiller pump 2 13 GPMCondenser 2 7 Foul factorEvaporator 2 6 Foul factorChiller 15 22 # & % powerSec. pump 6 17 kWSystem data 22Totals 46 126 8

The equations simultaneously solve for each hour of weather data arriving at a condition of energy balance where energy into the plant equals energy out of the plant for each hour.

Chapter 1 will show that the (SEE) Plant Model duplicates the data1 of a Salt Lake City plant operating with 100% tower fan speed.

System Energy Equilibrium (SEE) Plant Model Chapter 1-Salt Lake City Plant at 100% Tower Fan Speed

http://kirbynelsonpe.com/

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Wet Bulb (F) Evaporator Load (ton)

Figure 1: Salt Lake City Evaporator load & wet bulb temperature plus chiller kWFigure 1 gives the chiller kW1 as the evaporator load and wet bulb temperature drop, illustrating the wide variation in chiller kW. The (SEE) Plant Model must duplicate this performance and go to 50% tower fan speed and duplicate the performance by only changing the tower fan speed to 50%. This Chapter 1 will deal with 100% tower fan speed and Chapter 2 with 50% tower fan speed.

Chiller/Tower SelectionSufficient information is provided by Schwedler1 to define the chiller and tower used in the article. Tables 1 & 2 define the chiller and tower installed into the (SEE) Plant Model.

Design Load (Ton) 1000.0Chiller kW 508.0Chiller kW/ton .508Evap water flow rate (gpm) 2400.0Evap entering water temp (F) 54.0Evap leaving water temp (F) 44.0Evap refrigerant temp (F) 38.8Evap refrigerant approach (F) 5.2Evap pressure drop (ft. water) 26.9Evap water velocity (ft. /sec.) 10.06Cond heat rejection (ton) 1155Cond water flow rate (gpm) 3000.0Cond entering water temp (F) 76.0Cond leaving water temp (F) 85.26Cond refrigerant temp (F) 90.16Cond refrigerant approach (F) 4.9

Cond pressure drop (ft. water) 30.Cond water velocity (ft. /sec) 10.28

Table 1: Salt Lake City Chiller Design Data2

Tower Low cost tower1

Design Wet bulb 66.0FDesign Load 1155 tonTower water flow 3000 gpmApproach temperature 10.0 FCalculated range 9.26FRange + approach 19.26FTwo cells with 40HP fans1 64 kW1

Selected Tower3 NC8403TLS2Cold water 76FReturn water to tower 85.26FCapacity 100.5%Static lift 12.234 ft.

Table 2: Salt Lake City Tower Design Data3

Chiller Coefficient of Performance Ratio

The maximum COP is defined by the refrigerant temperatures.

Max COP = T2/ (T1-T2)

T2 = absolute evaporator refrigerant temperature.T1 = absolute condenser refrigerant temperature. From Table 1 for 100% tower fan speed at design conditions.

Max COP4 = (38.8+460)/(90.16+460-38.8+460) = 9.71

The actual COP of the chiller/tower is defined by cooling provided/power required. From Table 1.

Actual COP = (1000 ton * 12000 btu/ton) / (508kW * 3413 btu/kW) = 6.92

Therefore, the COP ratio at design is 6.92/9.71 = .7127

ALSO, kW/ton = 3.52/COPact = 3.52/6.92 = .508 kW/ton

This COP ratio is a necessary input to the (SEE) model & will vary due to type chiller and refrigerant used as illustrated by page 43.8 Table 1 of reference 6.

Design Conditions (SEE) Schematic

(SEE) Schematic 1 is the (SEE) Model of the design conditions of Tables 1 & 2. Nomenclature is given at the end of this document. Schwedler1 only gave kW values for the chiller and tower fan and the (SEE) Plant Model must include the P/S pumping and the condenser pumps. Note the plant load is 984.1 ton and the P/S pumps add heat to the system resulting in 1,000-ton evaporator load. The chiller motor adds heat to the refrigerant resulting in a condenser load of 1,153 ton and the tower exhausts 1,171 ton because of the tower fan kW.

Schematic 1 gives an energy balance showing an energy in = energy out of 1,176.6 ton.

Condenser >pumpT-kW = 35.2(cond)ton= 1153 Pipesize-in = 6.0 (H)T-pipe= 9.6 Tower

TCR= 90.11 > GPMT= 2994 > (ewt)T= 85.22 tfan-kW= 64.0TCR-app= 4.88 (H)T-total= 51.83 (H)T-static = 12.2 Tfan-kW= 64.0

(COND)ton= 1153 PT-heat ton = -1.70 < gpmT = 2994 tfan-%= 100%(H)cond= 30.0 <PumpT-kW= 35.22 < (lwt)T = 75.98 tton-ex= -1171

(cond)ft/sec= 10.28 EfTpump= 0.83 Tapproach = 9.98 T#= 1Ptower # = 1 Trange= 9.24 T-Ton-ex= -1171

Trg+app = 19.2Compressor

(chiller)kW= 508.0(chiller)lift= 51.26(chiller)%= 100% Salt Lake City -(chiller)#= 1 Chiller Plant

(CHILLER)kW= 508.0 Tdry bulb = -(chiller)kW/ton= 0.508 Twet bulb = 66.0Plant kW = 676.7

PlantkW/ton = 0.688 Chiller kW = 508Tower kW = 64

Pumps kW = 105 > Evaporator Plant kW = 677

(evap)ton= 1000.0 Ein = Eout (TON)TER= 38.85 Plant load= 984.1

TER-app= 5.200 Plant kW= 192.5 ^ EVAPton= 1000 Energy in= 1176.6

(H)evap= 26.90 Pump heat= -5.60(evap)ft/sec= 10.06 Tower exh.= -1171

(evap)des-ft/sec= 10.06 Energy out= -1176.6 ^ V

gpmevap= 2400.0 >pumpc-kW = 17.81 Psec-heat-ton = -2.94 GPMSEC = 2399.9Pchiller-# = 1 (lwt)evap = 44.048 > Psec-kW= 51.6 > (ewt)coil= 44.048

(H)pri-total= 31.91 v Efdes-sec-p = 0.80 ^ (H)pri-pipe= 1.1 Tbp= 44.048 Efsec-pump = 0.80 # Buildings = 1.0

(H)pri-fitings= 3.9 gpmbp= -0.11 (H)sec= 91.4 PLANTton = 984.1(Ef)c-pump= 0.81 (H)pri-bp= 0.00 (H)sec-pipe= 22Pc-heat-ton= -0.96 v (H)sec-bp= 0.00 Pipesize-in = 14.0

^ < Pumpc-kW= 17.81 (ewt)evap = 54.048 < (GPM)sec= 2399.9 < (lwt)coil= 54.048

(SEE) Schematic 1-Salt Lake City-Design Load at 100% Tower fan Speed.

Schematic 1 illustrates the modeled plant at design conditions agreeing with the mfg1 data. The following will show that the (SEE) Model agrees, within 3%, for all the data given1. First the tower data comparison.

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Figure 2: (SEE) Tower Model vs. Mfg1 Data

Figure 2 top chart illustrates that the tower leaving water temperature, or condenser entering water temperature, tracks close to the Mfg1 data. A necessary condition given the effect on condenser refrigerant temperature and therefore chiller lift that in large part defines chiller kW demand. The bottom chart of Figure 2 illustrates how the tower refrigerant approach and range vary with

condenser load and wet bulb. These numbers track close to Mfg3 data.

19.22 17.73 16.07 14.42 12.77 11.00 9.21 7.46

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Figure 3: Tower Model vs. Mfg1 data Figure 3 top chart gives the condenser entering water temperature, also given by the top chart of Figure 2, and the condenser leaving water temperature that establishes the condenser refrigerant temperature because of the condenser refrigerant approach temperature given by the secondary horizontal axis. Recognize that the condenser refrigerant temperature is in part defined by the chiller kW and the chiller kW is defined by the tower performance. The point is that the chiller/tower acts as a unit.

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Ref 1 Chiller (kW/ton) (SEE) Model Chiller (kW/ton)

Figure 4: Chiller/Tower Model vs. Mfg1 Data

Figure 4 top chart illustrates how close the model agrees with the Mfg1 chiller kW data and the bottom chart gives the kW/ton values with the secondary horizontal axis showing agreement within 2%. Figure 5 shows the refrigerant temperatures that must be modeled to define lift and therefore chiller kW.

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Figure 5: Refrigerant temperatures & Chiller Lift

The next figures illustrate that the model mimics the figure of ASHRAE Handbook6.

ASHRAE Handbook6

Figure 6: Figure copied from ASHRAE Handbook6.

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Salt Lake City Plant (SEE) Model-100% Tower fan Speed

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Figure 7: Modeled Chiller/Tower temperaturesThe axis of Figures 6 & 7 is reversed but clearly illustrate the Model agrees with the Handbook6 data configuration.

SEE Model Iterates to Study State Condition

Above we stated that the chiller/tower acts as a unit. The truth of this statement put a requirement on the plant model that is analogies15 to a feedback control system. Consider what happens when the plant load or wet bulb temperature changes. The chiller kW must increase or decrease to provide 44F supply water at this new condition. As the chiller kW changes the condenser load changes and therefore the tower cold water changes that changes the chiller lift and therefore the required chiller kW to give 44F supply water. In a real plant the system would iterate to steady state condition so therefore must also the plant model. This requirement on the plant model is, the author believes, a necessary capability that is provided here by a set of equations that iterate to steady state conditions after a change in conditions. The model equations do not use look up tables.

Plant Schematics The plant schematics were developed as part of the model to provide more complete understanding of the plant as conditions change. The following three schematics illustrate the plant performance at 800-ton, 500-ton, and 300-ton evaporator load.







(CHILLER)kW= 361 Tdry bulb = -(chiller)kW/ton= 0.451 Twet bulb = 63.0

Plant kW = 516PlantkW/ton = 0.655 Chiller kW = 360.7

Tower kW = 64.0Pumps kW = 91.3

> Evaporator Plant kW = 516.0(evap)ton= 800.0 Ein = Eout (TON)

TER= 40.3 Plant load= 787.7TER-app= 3.74 Plant kW= 146.8

^ EVAPton= 800 Energy in= 934.5(H)evap= 26.9 Pump heat= -5.32

(evap)ft/sec= 10.06 Tower exh.= -929(evap)des-ft/sec= 10.06 Energy out= -934.5

^ Vgpmevap= 2400 <pumpc-kW = 17.84 Psec-heat-ton = -2.7 GPMSEC = 1920Pchiller-# = 1 (lwt)evap = 44.01 > Psec-kW= 38.3 > (ewt)coil= 44.01


(H)pri-fitings= 3.9 gpmbp= -480 (H)sec= 80 PLANTton = 788(Ef)c-pump= 0.81 (H)pri-bp= 0.04 (H)sec-pipe= 14Pc-heat-ton= -0.96 v (H)sec-bp= 0.00 Pipesize-in = 14.0

^ < Pumpc-kW= 17.8 (ewt)evap = 52.01 < (GPM)sec= 1920 < (lwt)coil= 54.01

(SEE) Schematic-Salt Lake City-800-ton Load & 100% Tower fan Speed.

Nomenclature is given at the end so the reader can study the schematics to gain a better understanding how the plant performs at these given evaporator loads.The author has found plant schematics a necessary tool in understanding some characteristics and responses of the plant, especially multi chiller plants that we will get to at later chapters.








Plant kW = 343PlantkW/ton = 0.698 Chiller kW = 201

Tower kW = 64Pumps kW = 78

> Evaporator Plant kW = 343(evap)ton= 500.0 Ein = Eout (TON)




^ Vgpmevap= 2400 >pumpc-kW = 17.97 Psec-heat-ton = -2.53 GPMSec = 1200Pchiller-# = 1 (lwt)evap = 44.03 > Psec-kW= 24.7 > (ewt)coil= 44.0

(H)pri-total= 32.2 v Efdes-sec-p = 0.80 ^ (H)pri-pipe= 1.1 Tbp= 44.03 Efsec-pump = 0.64 # Buildings 1.0

(H)pri-fitings= 3.9 gpmbp= -1200 (H)sec= 70.0 PLANTton = 491(Ef)c-pump= 0.81 (H)pri-bp= 0.27 (H)sec-pipe= 6Pc-heat-ton= -0.97 v (H)sec-bp= 0.00 Pipesize-in = 14.0



All values given by the schematics and figures are consistent i.e., the same from one figure or schematic to the other.








Plant kW = 271PlantkW/ton = 0.926 Chiller kW = 134.1

Tower kW = 64.0Pumps kW = 73.3

> Evaporator Plant kW = 271.4(evap)ton= 300.0 Ein = Eout (TON)




^ Vgpmevap= 2400 <pumpc-kW = 18.11 Psec-heat-ton = -3.0 GPMSEC = 720Pchiller-# = 1 (lwt)evap = 44.03 > Psec-kW= 20.0 > (ewt)coil= 44.03


(H)pri-fitings= 3.9 gpmbp= -1680 (H)sec= 70 PLANTton = 293(Ef)c-pump= 0.81 (H)pri-bp= 0.54 (H)sec-pipe= 2Pc-heat-ton= -0.98 v (H)sec-bp= 0.00 Pipesize-in = 14.0



Chapter 2 will address the Salt Lake City plant at 50% tower fan speed and Chapter 3 will address the effect of the condenser & evaporator refrigerant approach temperatures effect on chiller/tower kW/ton.

Best Regards to All, Kirby Nelson P.E.Life member ASHRAE

References 1. Schwedler, Mick. July 1998 “Take It to

The Limit…Or Just Halfway?” ASHRAE Journal.

2. Manufacturer’s chiller selection.3. SPX Cooling Technologies (Marley).

UPDATE Version

4. Introduction to Thermodynamics and Heat Transfer. 1956 Prentice-Hall, Inc. by David A. Mooney.

5. Thermal Environmental Engineering third edition. 1998 Prentice-Hall Inc. by Thomas H. Kuehn, chapter 3.

6. 2012 ASHRAE HANDBOOK, HVAC Systems and Equipment, page 43.10 Figure 11 Temperature Relations in a Typical Centrifugal Liquid Chiller.

7. Mark Baker, Dan Roe, Mick Schwedler. ASHRAE Journal June 2006. “Prescription for Chiller Plants”.

8. ASHRAE Journal May 2016. “Modeled Performance Isn’t Actual Performance.”

9. Nelson, K. “Simulation Modeling of a Central Chiller Plant” CH-12-002. ASHRAE 2012 Chicago Winter Transactions.

10. Nelson, Kirby. July 2010 “Central-Chiller-Plant Modeling” HPAC Engineering

11. Nelson, Kirby. “System Energy Equilibrium (SEE) Building Energy Model Development & Verification”. http://kirbynelsonpe.com/

12. Liu, B. May 2011. “Achieving the 30% Goal: (copy and paste) https://www.energycodes.gov/sites/default/files/documents/BECP_Energy_Cost_Savings_STD2010_May2011_v00.pdf

13. Real weather data www.wunderground.com/

14. Taylor, S. 2011. “Optimizing design & control of chiller plants.” ASHRAE Journal (12).

15. Miles Ryan. ASHRAE Journal February 2021. “Understanding Cascade Control and Its Applications For HVAC”

Nomenclature

1. CENTRAL PLANT2. # Buildings = number of buildings served

by plant3. Plant ton = total load (ton) to plant

4. Primary/secondary pumping nomenclature

5. gpmevap = total gpm flow thru one evaporators

6. (H)pri-total = total primary pump head (ft) = (H)pri-pipe + (H)pri-fittings + (H)pri-bp + (H)evap

7. (H)pri-pipe = primary pump head due to piping (ft)

8. (H)pri-fittings = primary head due to pump & fitting (ft)

9. (Ef)c-pump = efficiency of chiller pump10. Pc-heat-ton = chiller pump heat to

atmosphere (ton)11. Pc-kW = one chiller pump kW demand

(kW)12. Pchiller-# = number chiller pumps operating13. (lwt)evap = temperature water leaving

evaporator (F)14. Tbp = temperature of water in bypass (F)15. gpmbp = gpm water flow in bypass16. (H)pri-bp = head if chiller pump flow in

bypass (ft)17. (ewt)evap = temp water entering

evaporator (F)18. Psec-heat-ton = secondary pump heat to

atmosphere (ton)19. Psec-kW = kW demand of secondary pumps20. Efdes-sec-p = design efficiency of secondary

pumping21. Efsec-pump = efficiency of secondary

pumping22. (H)sec = secondary pump head (ft) = (H)sec-

pipe + (H)sec-bp + (H)coil + (H)valve

23. (H)sec-pipe = secondary pump head due to pipe (ft)

24. (H)sec-bp = head in bypass if gpmsec > gpmevap

25. GPMsec = water gpm flow in secondary loop

26. (ewt)coil = water temperature entering coil (F)

27. Pipesize-in = secondary pipe size (inches)28. (lwt)coil = temperature of water leaving

coil (F)29. Evaporator30. (evap)ton = load (ton) on one evaporator31. TER = evaporator refrigerant temp (F)32. TER-app = evaporator refrigerant approach

(F)

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33. EVAPton = total evaporator loads (ton)34. (H)evap = pump head thru evaporator (ft)35. (evap)ft/sec = velocity water flow thru

evaporator36. (evap)des-ft/sec = evaporator design flow

velocity37. Compressor:38. (chiller)kW = each chiller kW demand39. (chiller)lift = (TCR – TER) = chiller lift (F)40. (chiller)% = percent chiller motor is

loaded41. (chiller)# = number chillers operating42. (CHILLER)kW = total plant chiller kW43. (chiller)kW/ton = chiller kW per evaporator

ton44. Plant kW = total kW demand of plant45. (Plant)kW/site ton = Plant kW per site ton46. Condenser nomenclature:47. (cond)ton = load (ton) on one condenser48. TCR = temperature of condenser

refrigerant (F)49. TCR-app = refrigerant approach

temperature (F)50. (COND)ton = total load (ton) on all

condensers51. (H)cond = tower pump head thru

condenser (ft)52. (cond)ft/sec = tower water flow thru

condenser53. Tower piping nomenclature54. Pipesize-in = tower pipe size (inches)55. gpmT = each tower water flow (gpm)56. GPMT = total tower water flow (gpm)57. (H)T-total = total tower pump head (ft)58. PT-heat = pump heat to atmosphere (ton)59. PT-kW = each tower pump kW demand60. EfT-pump = tower pump efficiency61. Ptower # = number of tower pumps62. (H)T-pipe = total tower pump head (ft)63. (ewt)T = tower entering water

temperature (F)64. (H)T-static = tower height static head (ft)65. Trange = tower range (F)= (ewt)T – (lwt)T

66. (lwt)T = tower leaving water temperature (F)

67. Tapproach = (lwt)T – (Twet-bulb)68. Tower nomenclature

69. tfan-kW = kW demand of one tower fan70. Tfan-kW = tower fan kW of fans on

71. tfan-% = percent tower fan speed72. tton-ex = ton exhaust by one tower

73. T# = number of towers on74. Tton-ex = ton exhaust by all towers on75. Trg+app = tower range + approach (F)

SYSTEM ENERGY EQUILIBRIUM (SEE) CHILLER PLANT ... · Web view2012 ASHRAE HANDBOOK, HVAC Systems and...

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