Changes in Pear CA Storage Rooms as Influenced by Evaporator Fan Operation
Fan motor speed control, through installation of variable frequency drives, allows infinite variation of shaft rotational speed (rpm). Basic fan laws (presented in the ASHRAE HVAC Systems and Equipment Handbook) show that air delivery (volumetric flow rate) is directly proportional to rotational speed and power is proportional to shaft rpm raised to the third power. Thus, if evaporator fan motor speed is reduced to one-half, air delivery will be one-half and power use by the fan motor(s) will be one-eighth of full rpm values.
Fruit storage warehouses that have adopted evaporator fan on/off cycling have two control strategies from which to choose. The simplest is to turn evaporator fans on and off at selected time intervals. On/off fan cycling can also be based on balancing the average fruit temperature as measured in several bins within the room to the room set point. When fruit temperature is at or very near the room set point, the fans cycle off. When the average fruit temperature rises above the allowed level, the evaporator fans are activated and operate until the desired fruit temperature is again reached and then they are turned off. This control strategy was successfully documented by Koca and Hellickson, 1993.
Comprehensive computer control of warehouse refrigeration systems may also include computer software and hardware to automatically adjust compressor and condenser pressures, control atmosphere gas mixes and power venting, and schedule defrost cycles. Ultimately, whatever combination of technologies is adopted, the modified system must not adversely affect fruit quality.
Precise temperature management within the storage space is critical to fruit quality preservation. Energy conservation practices that include reduction of air volumetric flow rate and velocity also affect changes in airflow patterns within the room. Warehouse operators need to be confident that the fan-cycling scheme chosen does not adversely affect fruit temperatures throughout the room. Undetected temperature changes may cause reductions in fruit quality that would be more costly than the realized energy savings. Therefore, this research was initiated with the following specific objectives:
- Compare fruit temperatures in a tight-stacked controlled atmosphere room to those in a conventionally stacked room during periods of operator managed on/off fan cycling.
- Compare overall fruit mass loss in each room.
Two nearly identical controlled atmosphere pear storage rooms at Duckwall-Pooley Fruit Company, Odell, Oregon, were used to compare effects of various fan-cycling schemes in conventionally stacked versus tight-stacked rooms. Rooms CA13 and CA16 each measured approximately 18.6 meters long, 9 meters wide and 7.6 meters high. In 1995-96 and 1996-97, CA 16 was loaded as normal with 14 stacks of bins front to back, 6 rows wide and 10 bins high. Total capacity was 825 bins. Room CA 13 was tight-stacked. Space between bin rows was eliminated; thus bin placement was 14 stacks front to back, 7 rows wide and 10 bins high in 1995-96 and 11 bins high in 1996-97. Total bin capacity in CA 13 was 945 bins in 1995-96 and 1049 bins in 1996-97. Hellickson and Baskins (1996) presented an illustration of bin placement and temperature monitoring locations in each room.
Each room is equipped with one Krack Model APPL 423 evaporator coil located above the door. The units have aluminum fins spaced 4 per inch and are rated at 23 tons of refrigeration capacity at 12 °F temperature difference. Each unit has four 2-hp, 1160-rpm fans that deliver a total of 50,700 cfm.
Results and Discussion
D'Anjou pears that had been pre-cooled and returned from presizing were placed in both rooms for the 1995-96 storage season. Room CA 13 was closed September 30th and CA 16 October 1. Full capacity evaporator operation was used for 11 days before fan cycling was initiated in each room. The two inner fans of the evaporator coil in each room were turned off October 11, 1995 to begin the energy conservation tests. Fan cycling, which included lowering the room set point from 30 °F to 29.7 °F and turning the fans on and off at various time intervals was initiated on October 18, 1995. Tables 1 and 2 list the fan-cycling test schedule, energy savings, room set-points, ammonia temperature changes measured at the evaporator coil inlet and exit, and maximum fruit temperature variations in the bins during each test.
Observations from the 1995-96 and 1996-97 tests include:
Changing from four fans on with no cycling to two fans on with no cycling caused the both ammonia inlet and exit temperatures to drop. Air temperatures entering the coil to increased and air temperatures exiting the coil decreased. Room relative humidity decreased. Fruit temperatures decreased. Some data were lost during this period, however, temperature changes were essentially the same as recorded for this operational mode during the 1996-97 season. Those data are presented below. Changing from two fans on continuously to two fans operating two hours on followed by two hours off plus reducing the room set-point to 29.7 °F, caused the ammonia exit temperature to further decrease from 27.25 to 26.25 °F. Air temperature entering the evaporator coil decreased slightly and the air exiting the evaporator showed a greater reduction in temperature. The air temperature split across the coil was increased. Fruit temperatures typically increased 0.1 to 0.2 °F, especially at the Bin 10 level.
Changing from two fans cycling at two hours on followed by two hours off to all four fans cycling two hours on then two hours off caused the ammonia exit temperature to increase 0.5 °F. Air temperature entering the coil cooled 0.25 °F while the air exit temperature remained nearly the same. Thus, the air temperature spilt across the coil was reduced to about 0.25 °F.
Changing from all four fans on and cycling at two hours on and two hours off to all fans on cycling at two hours on followed by four hours off caused a slight lowering of ammonia exit temperature (27.0 down to 26.5 °F) and a slightly greater split in air temperature change across the coil (from 0.25 to 0.3 °F). Fruit temperatures in the room slowly increased 0.1 to 0.2 °F causing the ammonia temperature to slowly decrease.
Cycling all of the fans on for two hours and off for six hours again caused a gradual decrease in refrigerant temperature from 26.75 to 26.5 °F. A slightly greater air temperature split across the coil from 0.2 to 0.3 °F and a gradual increase in fruit temperature of approximately 0.3 °F were recorded.
Field run fruit was placed in CA 13 and CA 16 for the 1996-97 storage season. The rooms were closed September 20th and energy conservation was initiated on September 29, 1996, when the inner two fans of the evaporator coil were turned off.
Changing from four fans on with no cycling to two fans on with no cycling caused the both ammonia inlet and exit temperatures to decrease approximately 0.7 °F from 26.5 to 25.8 °F. The air temperature entering the evaporator coil increased 30.2 to 30.5 °F. Air temperature exiting the coil decreased from 30.0 to 29.7 °F which increased the air temperature split across the coil from approximately 0.2 to 0.8 °F. Relative humidity, measured at the center of the room at top bin level decreased approximately 0.7% from 90.0% to 89.3%. The largest fruit temperature reduction (1.2 °F) was recorded in S14R7B10. Only one bin fruit temperature increased (0.1 °F) at S8R1B10.
The system was operated at this setting for twenty days at which time cycling the two operating fans was initiated with 4 hours on followed by 2 hours off. Reducing the room set point 0.3 °F caused an immediate drop in ammonia temperature exiting the evaporator coil of 1.5 °F which then came back up about 0.5 °F (27.0 °F to 25.5 °F then up to 26.0 °F). Air temperature entering the evaporator coil cooled from 30.3 °F to 30.15 °F. Air temperature exiting the evaporator dropped from 29.95 °F to 26.95 °F. Thus the air temperature split across the evaporator coil increased from 0.35 °F to 0.50 °F.
On November 20, 1996, all four fans were made operational with a 4 hour on/2 hour off fan cycling scheme. Turning on the two previously idle fans caused the ammonia temperature exiting the coil to increase from 26.5 °F to 27.3 °F. The air temperature split across the coil adjusted from 30.1 °F entering and 29.65 °F exiting to 29.9 °F exiting and 29.8 °F entering. Therefore, the split across the coil was reduced from 0.4 °F to essentially 0 °F for the next four days. The temperature split across the coil was minimal at 0.1 °F or less for the remainder of this test. Relative humidity increased approximately 1.5% (to 91.5%) immediately after the operational change on November 20, 1996, but declined within 48 hours to 89.5 - 90.0% which was nearly 0.5% below levels recorded during the two fan 4-hour on /2-hour off schedule. Continuous measurement of fruit weight indicated a 2.5 gram increase in mass during the 12 hours immediately after the two inner fans were turned on.
On December 20, 1996, refrigerant temperature dropped from 27.0 to 26.2 °F when the inner two fans in the evaporator coil were again turned off. The air temperature spit across the coil increased from 0.1 °F (29.9 °F exit temperature) to 0.4 °F with an ammonia exit temperature of 29.7 °F. Relative humidity immediately dropped 1%, but recovered to 90.5 - 91.0% within four hours.
Table 3 lists the fan cycling schemes, energy savings, room set points, ammonia exit temperature changes and bin fruit temperature variations that were recorded. No fan cycling was scheduled in CA 16. Ammonia exit temperature remained nearly constant at 26 °F. Air temperature entering the evaporator coil was approximately 30.0 °F and the split was 0.25 to 0.3 °F. Bin fruit temperature variations were very slight. The maximum difference in bin temperatures on the floor was 0.4 °F, at mid-level (Bin 6) was 0.4 °F and 0.5 °F in the top bins.
Summary and Conclusions
Operator controlled fan cycling is a cost-effective method of achieving energy conservation in fruit storage rooms. Operational experience indicates that cycling fans two hours on followed by either four hours off or six hours off caused a gradual increase in fruit temperatures accompanied with a gradual decrease in refrigerant temperature. Relative humidity increases and decreases directly correlated to refrigerant temperature changes. Cycling fans two hours on and 2 hours off or four hours on followed by two hours off maintained essentially flat fruit temperatures once the ammonia temperature change caused by the management scheme change leveled off.
Changing from four fans operating to two fan operating caused refrigerant temperatures to decrease. This increase is primarily due to room air being circulated through essentially half of the coil.
Once fruit have reached a stable storage temperature, evaporator defrost periods can be reduced. Every other day coil defrost appeared sufficient in CA 16 which was loaded with 825 bins of fruit. The additional fruit in CA 13 during the 1996-97 season (1049 bins) did not allow less than daily defrosting of the evaporator coil.
ASHRAE. 1992. ASHRAE HVAC Systems and Equipment. Atlanta, GA: Am. Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
Koca, R. W. and M. L. Hellickson: 1993. Energy savings in evaporator fan-cycled apple storages. APPLIED ENGINEERING IN AGRICULTURE 9:6(553-560). Am. Society of Agric. Engineers. 2950 Niles Rd., St. Joseph, MI 49085.
Hellickson, M. L. and R. A. Baskins. 1996. Cool-down comparisons of d'Anjou pears tight-stack versus conventional stacking. Or. AES Tech. Paper No. 10,956. Proceedings of the 12th Annual Washington Tree Fruit Postharvest Conference. Wash. State Horticultural Association. P.O. Box 136, Wenatchee, WA. 98807. March 1996.
Table 1. Fan Cycling Schemes in CA 13 for 1995-96 Storage Season.
|Beginning Test Date||Active Fans||Fan - Cycling hrs on / hrs off||Energy Savings||Room Set Point (°F)||Ammonia Exit dT at Coil (°F)||Max Bin Temp Variation1 (°F)|
|10/11/95||Outer two||None||50%||30.0||27.5 - 27.2||Some still cooling|
|10/18/95||Outer two||2 on / 2 off||75%||29.7||27.25 - 26.25||+ 0.2, - 0.5|
|11/01/95||All 4 fans||2 on / 2 off||50%||29.7||26.25 - 26.75||+ 0.1, - 0.5|
|11/09/95||All 4 fans||2 on / 4 off||66.7%||29.7||27.0 to 26.75||Slowly + 0.2|
|11/16/95||All 4 fans||2 on / 6 off||75%||29.7||26.75 to 26.5||Slowly + 0.3|
|11/22/95||All 4 fans||2 on / 4 off||66.7%||29.7||26.5 to 26.7||- 0.2|
|11/29/95||All 4 fans||None||0%||30.0||27.7||- 0.5|
|12/06/96||All 4 fans||2 on / 4 off||66.7%||29.7||27.4 to 26.6||+0.8|
|12/13/95||Outer two||None||50%||30.0||27.0 to 26.5||+0.1, -0.7|
|12/20/95||All 4 fans||None||0%||30.0||27.3||-0.2|
1Maximum temperature variation is the amount fruit temperature changed in any one bin. Maximum temperature variation was most often recorded in bins on the top.
Table 2. Fan Cycling Schemes in CA 16 for 1995-96 Season.
|Beginning Test Date||Active Fans||Fan Cycling Hrs on/Hrs off||Energy Savings||Room Set Point (°F)||Ammonia Exit dT at Coil (°F)||Max Bin Temp Variation (°F)|
|10/11/95||Outer two||None||50%||30.0||26.5 to 27.5*||---|
|10/18/95||Outer two||2 on/ 2 off||75%||29.7||26.0||+0.3, -0.1|
|11/01/95||All 4 fans||2 on/ 2 off||50%||29.7||26.5||+0.1, -0.4|
|11/09/95||All 4 fans||2 on/ 4 off||66.7%||29.7||26.5||+0.3|
|11/16/95||All 4 fans||2 on/ 6 off||75%||29.7||26.5||+0.5|
|11/22/95||All 4 fans||2 on/ 4 off||66.7%||29.7||26.5||-0.3|
|11/29/95||All 4 fans||None||0%||30.0||27.0||-0.7, +0.1|
|12/06/96||All 4 fans||2 on/ 4 off||66.7%||29.7||26.5||+0.6|
|12/13/95||Outer two||None||50%||30.0||25.5 to 26.5||+0.8, -1.1|
|12/20/95||All 4 fans||None||0%||30.0||27.0 to 27.5||-0.5|
*This was not a typical result. Changing from two defrost periods per day to one on this date caused an abnormal situation.
Table 3. Fan Cycling Schemes in CA 13 and CA 16 for the 1996-97 Storage Season.
|Beginning Test Date||Active Fans||Fan Cycling Hrs on/Hrs off||Energy Savings||Room Set Point(°F)||Ammonia Exit dT at Coil (°F)||Maximum Bin Temp Variation (°F)|
|09/26/96||Outer two||None||50%||30.0||26.5 to 26.0||-1.2, +0.1|
|10/18/96||Outer two||4 on/ 2 off||66.7%||29.7||26.5 to 25.75||-0.3|
|10/20/96||All 4 fans||4 on/ 2 off||33.3%||29.7||26.5 to 27.25||+0.3, -0.1|
|12/20/96||Outer two||4 on/ 2 off||66.7%||29.7||27.0 to 26.5||-0.2|
Dr. Martin L. Hellickson(1), P.E., Associate Professor and Robert A. Baskins(2), Refrigeration Manager
(1)Oregon State University
Department of Bioresource Engineering
Gilmore Hall, Corvallis, OR 97331-3906
(2)Duckwall-Pooley Fruit Company
3430 Davis Dr., Hood River, OR 97031
14th Annual Postharvest Conference,
March 10-11, 1998