Case Study of Chilled Water Loop Low DT Fault Diagnosis

Transcription

1 Case Study of Chilled Water Loop Low DT Fault Diagnosis Presented by Lei Wang Ph.D. P.E. Energy Systems Laboratory, Texas A&M University System Sep. 15, 2014 p. 1

2 Outline Introduction Methodology and Investigation Plan Data Collection and Analysis Casual Factors Analysis Roots Causes of Low ΔT Overall Impact of Observed Issues on CHW ΔT Conclusions p. 2

3 Introduction Energy impact of degrading Chilled Water Delta-T Q Btu h = 500 GPM T Low ΔT reduce chiller actual capacity Low ΔT wastes energy by requiring additional chillers to operate and pumps to circulate more water than necessary p. 3

4 Introduction ChW ΔT Baseline of Case Study Project Design ΔT p. 4

5 Introduction Facility and Chilled Water Systems Building Area: 633,833 ft 2 (58,203m 2 ) Chillers Capacity : 4125 tons (14,507 kw) (4 chillers+1 WSHP) Pumps: 5 primary pumps (95 HP) ; 3 secondary pumps (225 HP) Airside Systems: 23 SZ-CAV; 26 SZ-VAV; 10 SDVAV; 17 OAHUs; 3 HC-OAHU; 3 ACU; 72 FCUs. Total 153 units p. 5

6 Methodology and Investigation Plan A typical root-cause analysis process is depicted as following: p. 6

7 Data Collection and Analysis BAS Trending data p. 7

8 Data Collection and Analysis Logger Deployment And Sensor Verification p. 8

9 Data Collection and Analysis Spot Measurements and Design Information Branch Logger Number Measured ΔT 1 ΔT at Design Conditions 2 Total CHW Flow at Design Conditions [ºF] [ºC] [ºF] [ºC] [gpm] [L/s] Branch Branch Branch Total Branch 3-1 NA NA Branch 3-2 NA NA Branch Branch p. 9

10 Data Collection and Analysis p. 10

11 Casual Factors CHW Loop Design Δt Analysis Unit Type SZ- CAV SZ- VAV SDVAV OAHU Munters OAHU ACU FCU Combin ed Design water flow GP M L/s Weighte d Average ΔT F C p. 11

12 Casual Factors CHW Supply Temperature Impact Analysis Design CHWST 42 F (5.56 C) 63% coils: 42.5 F (5.83 C) 37% coils: 42 F (5.56 C) CHWST: 45 F (7.22 C) p. 12

13 Casual Factors Chilled Water Coil Performance DAT Greater Than Design + Low ΔT DAT Less Than Design + Low ΔT 18 Sampled Units CHW ΔT Problem No CHW ΔT Problem Count Avg. CHW ΔT [ F] Avg. CHW ΔT [ C] p. 13

14 Roots Causes of Low ΔT Sub-optimal control strategies and set points for air handlers and fan coil units SZ and FCU units using space temperature control CHW valves Valve & Control Malfunctions Non-responsive chilled water control valve; Leaking by Valve; Chiller plant / cooling coil design parameter mismatch Chiller design DT 14 F(7.78 C); Coil design DT F (7.64 C); About 37% coils design EWT is 42 F(5.56 C)- Heat gain?; Good practice would allow for some degree of CHWST reset; p. 14

15 Overall Impact of Observed Issues on CHW ΔT Summer vs. Winter ΔT Impacts Issues Off-Design (OAT F) CHW ΔT Impact Design Conditions* Off-Design (OAT C) Design Conditio ns* SZ-VAV units with space T as control valve process variable 0.8 F 2.7 F 0.44 C 1.5 C Malfunctioning control valves 1.9 F 0.9 F 1.1 C 0.5 C SZ-CAV units with space T as control valve Process Variable 0.1 F 0.5 F 0.06 C 0.3 C Coil Design Mismatch 0 F 0.25 F 0 C 0.14 C Elevated CHWST in Winter 2.9 F F 0 F 1.6 C C 0 C Overall ΔT Impact F ~4.4 F 3.1 C C ~2.4 C Expected CHW ΔT without corrective action 7.5 F F ~9.6 F 4.2 C C ~5.3 C p. 15

16 Conclusions Three primary causes of the chilled water low ΔT for the case study facility 1. Sub-optimal control strategies and set points for air handlers and fan coil units 2. Valve & Control Malfunctions 3. Chiller plant / cooling coil design parameter mismatch Winter:5.5 F 6.5 F (3.06 C-3.61 C) Improvement Summer:4 F (2.22 C) Improvement 30% p. 16