Development of an Energy Impact Model for RTU Economizer Faults

Transcription

1 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, / 15

2 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, / 15

3 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, / 15

4 Economizer Fault Impacts Mixed Air Return Air Outdoor Air Return Air Recirculation Economizer faults impact system in several ways cooling capacity ventilation load cycle efficiency sensible heat ratio These contribute to longer run-times more energy discomfort A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15 ODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

5 Stuck Damper Impact on Mixed-Air ma ra Drybulb Temperature [ C] oa Humidity Ratio [g/kg-d.a.] Economizer modulates damper to control ventilation During warm outdoor-air conditions, only the minimum ventilation requirement should provided When the damper is stuck open, a larger fraction of outdoor-air is brought into the RTU This causes the mixed-air enthalpy to increase A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

6 Stuck Damper Impact on Mixed-Air Drybulb Temperature [ C] ra oa ma Humidity Ratio [g/kg-d.a.] Economizer modulates damper to control ventilation During warm outdoor-air conditions, only the minimum ventilation requirement should provided When the damper is stuck open, a larger fraction of outdoor-air is brought into the RTU This causes the mixed-air enthalpy to increase A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

7 Impact on Cooling Cycle Cycle impacts include higher evaporation temperature minimal impact on condensing temperature reduced pressure ratio Temperature [K] Normal Fault Entropy [kj/kg-k] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

8 Determining Energy Impact of Fault Use virtual sensors to estimate energy required to meet a cooling load Determine the relative impact on energy using models of normal performance W elec = Ẇcomp t load = Q cool COP t load where t load is the run-time required to meet the total cooling load. r elec = W elec,actual = W elec,normal ( Q cool t load /COP) ( actual Q cool t load /COP) = r cool r COP r run normal A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

9 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 r run = actual (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, / 15

10 Fault Impact Ratios Use virtual sensors to measure actual performance and normal models to estimate fault-free performance. r cool = Q cool,actual Q cool,virtual r COP = COP actual COP virtual r SHR = SHR actual SHR virtual r vent = Q vent,actual Q vent,virtual A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

11 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 , 30, 50, 70 Condition B , 40, 60, 80 Condition C , 33, 50, 67, 100 A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

12 sa ma ra Normal 5 Fault Drybulb Temperature [ C] oa Humidity Ratio [g/kg-d.a.] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

13 Capacity and Efficiency Impacts T oa = C, φ oa = 40% T oa = C, φ oa = 50% T oa = C, φ oa = 50% rcool[-] OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

14 Capacity and Efficiency Impacts T oa = C, φ oa = 40% T oa = C, φ oa = 50% T oa = C, φ oa = 50% rcop[-] OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

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, / 15

16 Energy Impacts T oa = C, φ oa = 40% T oa = C, φ oa = 50% T oa = C, φ oa = 50% relec[-] OAF [-] A. Hjortland (Purdue University) RTU OAE Fault Impacts July 15, / 15

17 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, / 15