Implications of GHP Fan Control Strategies on Humidity Control in Commercial Applications ASHRAE Winter Meeting February 9, 2005 Hugh I. Henderson, Jr., P.E. CDH Energy Corp. Cazenovia, NY DOE/NETL Project #DE-FC26-01NT41253
Fan Control Affects Latent Performance How is ventilation air supplied? Provided directly to heat pump inlet Provided directly to space If supply fan operates continuously, then latent capacity degrades at part load
Steady-State Cooling Coil Performance AC coil performance is well understood at steady-state conditions GHP is similar to conventional AC Coils provide both sensible cooling and moisture removal (latent cooling) SHR: sensible heat ratio..fraction of total cooling that is sensible Colder coil surfaces remove more moisture Reduced air flow provides more moisture removal (for DX coils) Higher condenser temperatures increase SHR i.e., high ground loop temperatures
Occurences Occurences Equipment Survey Reviewed specifications for 500 commercial and residential AC units Goal was to determine range of common coil geometries Variation by equipment type Typical DX AC coil is 3-rows, 15 fpi 180 160 140 120 100 80 60 40 20 0 200 180 160 140 120 100 80 60 40 20 0 Packaged Air Handlers Evap Coils 2 3 4 Number of Rows Packaged Air Handlers Evap Coils 10 11 12 13 13.5 14 14.5 15 16 17 18 Fin Spacing (fpi)
Sat. Temp = 42.4 Absolute Humidity Ratio (lb/lb) Typical AC Coil Performance Total: 2.0 tons (coil) Power: 2.5 kw EER: 9.6 Btu/Wh Sweetser - Upstairs Unit 0.020 0.015 0.010 SHR: 0.80 0.005 40 45 50 55 60 65 70 75 80 85 Dry Bulb Temperature (F) 0.000
Part Load Performance GHP cycles compressor ON and OFF based on a space thermostat The portion of time the coil operates (i.e., the runtime fraction) is longer when cooling loads are greater RTF = ON (ON + OFF) How do sensible and latent capacity vary under cyclic conditions?
Capacity (MBtu/h) Sensible and Latent Capacity With Continuous Supply Air Fan Operation 30 COIL1_TEST_4B_10B_16B_22B 08/30/02 07:42:04 Cycle #1 (Comp ON time: 45.0 minutes) Sensible 20 Transition pt Off-Cycle Evaporation is Adiabatic Process: Sensible Latent 10 0 Latent Removal -10 Compressor Latent Addition -20 0 20 40 60 80 100 time (minutes)
Latent Capacity Latent Degradation Concepts t wet = M o /Q L M o Useful Moisture Removal Q L = Q e /Q L Q e Evaporation Delay Time (t o ) time for first condensate to fall from pan M o Evaporation 0 20 40 60 TIME (minutes)
LHR Degradation Model Borrowed approaches used to develop partload efficiency degradation function for SEER test procedure Used same part-load assumptions (C d ): AC at startup described by a time constant Cycling rate driven by thermostat curve Additional latent assumptions: Coil surfaces hold a fixed amount of moisture (M o ) Off-cycle coil is like an evaporative cooler
SHR Comparing LHR Model to Field Tests Henderson (1998) compared the model results to field data collected on a 3-ton residential geothermal heat pump 1.10 1.00 0.90 0.80 0.70 Continuous supply air fan operation Measured Data: averaged into 4 hour intervals entering RH: 57% to 63% entering air temp: 68F to 72F loop temps: 65F to 90F 0.0 0.2 0.4 0.6 0.8 1.0 Runtime Fraction (-) Latent Degradation Model with: t wet = 720 sec = 1.07 N max = 3.6 cyc/h = 75 sec linear decay Steady State SHR = 0.76
Capacity (MBtu/h) Actual Laboratory Results 25 20 COIL2_TEST_4B_10B_16B_22B_25B 11/14/02 10:07:57 Cycle #2 (Comp ON time: 45.0 minutes) Sensible Integrated Moist (delay of 1.0 min) Mass twet QS= 21.8 Sens (lb & min): 1.98 17.0 Lat (lb & min): 1.97 16.9 15 10 (+) Latent (psych) Condensate 11.9 (gamma = 1.60) 5 QL= 7.4 QC= 6.7 (-) Latent (psych) Integration Pt 0 16.3 min 0 20 40 60 80 100 79.9 F, 60.4 F dp, 51.5 % Run 4 1.5 hz, 76.6 psi, 967 cfm, 30.04 in Hg time (minutes)
Sensible Heat Ratio (-) Laboratory Testing SHR Degradation LHR Model matches measured data at nominal entering conditions (80F db, 60.4F dp) 1.0 Constant Fan Cyclic Tests for COIL2 (80db, 60dp) Constant Fan Mode 1st Cycle 2nd Cycle 3rd Cycle 0.8 twet= 17.3, gamma= 1.50, exp Steady State SHR = 0.755 (based on condensate) 0.0 0.2 0.4 0.6 0.8 1.0 Runtime Fraction (-)
Sensible Heat Ratio (-) Laboratory Testing SHR Degradation (cont.) Model also matched measured data at other entering conditions! 1.00 0.90 Constant Fan Cyclic Tests for COIL2 (75db, 64dp) Constant Fan Mode (75F db, 64F dp) 0.80 1st Cycle 2nd Cycle 3rd Cycle 0.70 0.60 Steady State SHR = 0.533 (based on condensate) twet= 11.2, gamma= 0.49, exp 0.50 0.0 0.2 0.4 0.6 0.8 1.0 Runtime Fraction (-)
Sensible Heat Ratio (-) Laboratory Testing SHR Degradation (cont.) Coil 5 showed impact in AUTO Fan Mode! 1.0 AUTO Fan Mode Auto Fan Cyclic Tests for COIL5 0.8 1st Cycle 2nd Cycle 3rd Cycle 4th Cycle 5th Cycle Steady State SHR = 0.680 (based on condensate) 0.6 0.0 0.2 0.4 0.6 0.8 1.0 Runtime Fraction (-)
Sensible Heat Ratio (-) Field Testing SHR Degradation Significant AUTO fan degradation! 1.0 Sweetser - Downstairs AC Unit Steady State SHR = 0.855 0.8 Auto Fan Mode Constant Fan Mode (08/18/02 to 09/04/02) (Model: twet=15 min, gamma=2.2, Nmax=3) 0.0 0.2 0.4 0.6 0.8 1.0 Runtime Fraction
Measured Performance Parameters Coil 1 (Slanted slab, 3 row, 13 fpi, plain fins, orifice) Coil 2 Normal Flow (A-coil, 3 rows, 15.5 fpi, lanced sine-wave fins, TXV) Coil 2 Low Flow (A-coil, 3 rows, 15.5 fpi, lanced sine-wave fins, TXV) Coil 4 (vert. slab, 2 rows, 14 fpi, wavy fins, orifice) Coil 5 (slanted slab, 4 rows, 12 fpi, wavy fins, orifice) Coil 6 (A-coil, 3 rows, 13 fpi, wavy fins,txv) Coil 6 High Flow (A-coil, 3 rows, 13 fpi, wavy fins,txv) Coil 8 Chilled Water (vert. slab, 4 rows, 10 fpi, wavy fins,46f CHW) Notes: 1-2- 3- Size Fin Surface Area Retained Moisture Mass Cond Delay Time t wet (tons) (ft 2 ) (lb) (lb/kft 2 ) (min) (min) 2.9 243.8 2.1 8.6 13.5 16.5 2.4 237.8 2.0 8.4 16.3 17.0 1.4 237.8 2.0 8.4 32.5 29.0 1.8 137.4 1.9 13.8 23.5 18.5 2.3 162.7 1.4 8.6 11.5 9.5 1.7 231.1 2.7 11.7 34.0 33. 2.0 231.1 2.7 11.7 27.0 27. 1.5 135.0 1.4 10.4 26.5 26.5 Cooling capacity includes sensible and latent cooling at nominal conditions. Nominal conditions correspond to ASHRAE Test A test point. Surface area is gross fin area (coil face area x coil depth x fin spacing x 2). Condensate delay time and t wet are at nominal conditions.
Lab + Field: Retained Moisture Henderson (1998) Estimated in Field CHW Coil 2.9 ton Rooftop 10 tons Residential 3.5 ton Lab - Coil 8 Lab - Coil 7 Lab - Coil 6 Lab Measurements Lab - Coil 5 Lab - Coil 4 Lab - Coil 3 Lab - Coil 2 Lab - Coil 1 Korte & Jacobi (1997) - 2 4 6 8 10 12 14 16 18 20 Coil Moisture Retention (lb per 1000 ft2) Moisture per total gross fin area (gross area = face area x depth x fin spacing x 2)
Indoor Humidity (gr/lbm) Field Testing Impact of Constant Fan Large penalty for constant fan operation 100 80 Heink - Humidity Inside vs. Outside 60 Even for a dual-capacity AC unit 40 20 Auto Fan Mode Constant Fan Mode 0 0 50 100 150 Outdoor Humidity (gr/lbm)
Space Humidty (%RH) Impact on Space Humidity Levels 80 75 70 65 60 55 50 Room Humidity - July 13 Constant Fan AUTO Fan 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hour of Day TRNSYS Model of Hotel Room DX AC cycling in Hawaii DX AC
What Control Strategies Help? Do fan delay strategies help? No not until about a 10 minute delay Two-speed or staged capacity heat pumps? Yes increases runtime fraction (depends on coil split & fan control) Reduced airflow during off cycle? Yes would minimize off cycle evaporation Cycle the fan with the compressor? Yes whenever possible
Summary GHPs are like Air Conditioners, only they re geothermal Follow same steady state latent/sensible trends as conventional DX coil Subject to the same part load effects With constant fan, latent removal disappears below half load To maintain latent capacity Cycle fan with the compressor (when possible) Slow the fan down during off cycle Add more capacity stages