Development of an Energy Impact Model for RTU Economizer Faults Andrew L. Hjortland James E. Braun School of Mechanical Engineering Purdue University July 15, 2014 A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 1 / 15
Why Economizer FDD is Important ◮ Economizer faults are common in the field ◮ 215 RTUs studied, 62 % had failed economizers (Jacobs 2003) ◮ 503 RTUs studied, 64 % had economizers needing service (Cowan 2004) ◮ Some common problems ◮ stuck dampers ◮ dampers not fully modulating ◮ outdoor-air temperature (or enthalpy) sensor out of calibration ◮ improper change-over set point ◮ Faults may cause 10 % to 30 % more annual energy (Katipamula 2011) ◮ California Title 24 economizer requirements ◮ newly installed 4.5 ton or greater RTUs must have economizer ◮ must have economizer FDD system A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 2 / 15
Determining Fault Impacts is also Important Fault impacts required for optimal service recommendation ◮ significant impact on energy usage ◮ service and maintenance is expensive Need methodology to assess the severity of economizer faults ◮ Economic performance degradation index (EPDI) (Li 2009) ◮ Virtual sensors for AFDD (Li 2009, Kim 2013) A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 3 / 15
Economizer Fault Impacts Economizer faults impact system in several ways ◮ cooling capacity Mixed Air Outdoor ◮ ventilation load Air ◮ cycle efficiency ◮ sensible heat ratio These contribute to Return Air ◮ longer run-times Recirculation ◮ more energy Return ◮ discomfort Air A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 4 / 15
Stuck Damper Impact on Mixed-Air Economizer modulates damper to control ventilation Humidity Ratio [g/kg-d.a.] 30 ◮ During warm outdoor-air 25 conditions, only the 20 minimum ventilation oa requirement should 15 provided ma 10 ra ◮ When the damper is 5 stuck open, a larger fraction of outdoor-air is 0 − 10 0 10 20 30 40 50 60 brought into the RTU Drybulb Temperature [ ◦ C] ◮ This causes the mixed-air enthalpy to increase A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 5 / 15
Stuck Damper Impact on Mixed-Air Economizer modulates damper to control ventilation Humidity Ratio [g/kg-d.a.] 30 ◮ During warm outdoor-air 25 conditions, only the 20 minimum ventilation oa ma requirement should 15 provided 10 ra ◮ When the damper is 5 stuck open, a larger fraction of outdoor-air is 0 − 10 0 10 20 30 40 50 60 brought into the RTU Drybulb Temperature [ ◦ C] ◮ This causes the mixed-air enthalpy to increase A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 5 / 15
Impact on Cooling Cycle 350 Normal 340 Fault Temperature [K] Cycle impacts include 330 ◮ higher evaporation 320 temperature 310 300 ◮ minimal impact on 290 condensing temperature 280 ◮ reduced pressure ratio 270 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 Entropy [kJ/kg-K] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 6 / 15
Determining Energy Impact of Fault Use virtual sensors to estimate Determine the relative impact on energy required to meet a cooling energy using models of normal load performance W elec = ˙ W comp ∆ t load r elec = W elec , actual W elec , normal ˙ Q cool = COP ∆ t load � � ˙ Q cool ∆ t load / COP actual = where ∆ t load is the run-time required � � ˙ Q cool ∆ t load / COP to meet the total cooling load. normal = r cool r run r COP A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 7 / 15
Run-time Impact of Stuck Damper Faults Run-time can be determined from the total cooling load and equipment capacity ∆ t load = Q load ˙ Q cool = Q space + Q vent ˙ Q cool The relative increase in run-time caused by the fault can be defined as � � Q load / ˙ Q cool actual r run = � � Q load / ˙ Q cool normal = (1 − x vent , norm ) r space + x vent , norm r vent r SHR r cool A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 8 / 15
Fault Impact Ratios Use virtual sensors to “measure” actual performance and normal models to estimate fault-free performance. ˙ Q cool , actual r cool = ˙ Q cool , virtual r COP = COP actual COP virtual r SHR = SHR actual SHR virtual ˙ Q vent , actual r vent = ˙ Q vent , virtual A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 9 / 15
Laboratory Testing and Analysis 4-ton RTU installed in psychrometric chambers ◮ integrated economizer with control overridden ◮ scroll compressor ◮ TXV installed ◮ R410A T oa [ ◦ C] φ oa [%] T ra [ ◦ C] φ ra [%] Damper Position [%] Condition A 31.50 0.40 26.00 0.50 10, 30, 50, 70 Condition B 31.50 0.50 26.00 0.50 20, 40, 60, 80 Condition C 37.78 0.50 26.00 0.50 0, 33, 50, 67, 100 A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 10 / 15
30 Humidity Ratio [g/kg-d.a.] 25 20 oa 15 ma 10 ra sa Normal 5 Fault 0 − 10 0 10 20 30 40 50 60 Drybulb Temperature [ ◦ C] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 11 / 15
Capacity and Efficiency Impacts 1 . 15 T oa = 31 . 50 ◦ C, φ oa = 40% T oa = 31 . 50 ◦ C, φ oa = 50% T oa = 37 . 78 ◦ C, φ oa = 50% 1 . 10 r cool [-] 1 . 05 1 . 00 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 12 / 15
Capacity and Efficiency Impacts 1 . 15 T oa = 31 . 50 ◦ C, φ oa = 40% T oa = 31 . 50 ◦ C, φ oa = 50% T oa = 37 . 78 ◦ C, φ oa = 50% 1 . 10 r COP [-] 1 . 05 1 . 00 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 12 / 15
What’s the Problem? ◮ With damper stuck open, ◮ more capacity ◮ greater efficiency ◮ These improvements are negated by ◮ much greater ventilation load ◮ much longer run-time A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 13 / 15
Energy Impacts 1 . 8 T oa = 31 . 50 ◦ C, φ oa = 40% T oa = 31 . 50 ◦ C, φ oa = 50% 1 . 7 T oa = 37 . 78 ◦ C, φ oa = 50% 1 . 6 1 . 5 r elec [-] 1 . 4 1 . 3 1 . 2 1 . 1 1 . 0 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 14 / 15
Conclusions ◮ Methodology to estimate stuck damper fault impacts has been proposed ◮ Outputs can use virtual sensors from AFDD and be used in EPDI ◮ Future Work ◮ Identify minimum sensor requirement ◮ Evaluate multiple fault scenarios ◮ Propose criteria for optimal service scheduling A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, 2014 15 / 15
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