Evaporator Fan VSD's: Fruit Quality Considerations
There has been a great deal of interest in the use of variable speed motor drives (VSD) on evaporator fans in CA rooms in recent years as an energy saving measure. A number of installations have been completed in CA rooms in the state of Washington.
It has been well established that VSD, when applied to evaporator fans in CA rooms, can result in substantial energy savings. I would like to discuss the potential effect of the VSD applications on fruit quality, and my concerns about the way they are being applied.
In theory, the use of VSD in fruit CA storage is potentially the best thing that can happen to the fruit in terms of reducing fruit shrinkage. I will explain why that is true, using the results of my CA storage computer model that I have used for several years.
For my theoretical work, I assume that the air distribution through the CA room is fairly uniform, with relatively even cooling conditions for all the fruit.
Important relationships to be remembered in analyzing VSD's on evaporator fans are:
Airflow is proportional to fan speed. (CFM proportional to RPM)
Fan heat and fan energy is proportional to the third power, or "cube", of RPM.
Evaporator capacity is proportional to fan speed.
Air "throw" is proportional to fan tip speed.
Figure 1 shows the heat loads in a CA room of approximately 1,600 bins of Red Delicious apples, on a monthly basis during the CA season. This graph assumes the fruit is loaded in August and removed the following July. This graph does not include field heat removal, or "pull-down". The bottom curve is room heat, consisting of the heat transferred through the ceiling, walls, and floor of the room. The second curve up is the total of fan heat and fruit respiration heat at modern CA conditions. The top curve is total heat handled by the evaporator, including fan heat with the fans operating at 100 percent of their rated speed, which is needed during pull-down for adequate fruit temperature reduction.
You can see that fan heat is a large portion of the total evaporator load, particularly as the weather gets colder.
The third curve from the bottom is the total room heat if the fans are run at 50 percent of their rated speed all season. Please don't do this or your fruit will cool very slowly and lose quality significantly during pulldown. However, you can see that the evaporator heat load is much lower, and your energy bill will be greatly reduced.
Figure 2 shows the theoretical shrinkage the fruit (Red Delicious apples) would experience at both the 100 percent fan speed and 50 percent fan speed. I have also shown a shrinkage curve for my "optimum" fan speed profile, which starts at 100 percent, then slowly reduces to 50 percent by December, and increases again in April through July. It is apparent that shrinkage can be reduced, by reducing fan speed after pull-down is achieved.
The reason for the potential reduction in shrinkage by reducing fan speed is simply that the motor heat load is greatly reduced at lower fan speeds, which raises the evaporator, or refrigerant, temperature required to maintain the room "set point", which results in higher humidity in the CA room.
Excess fan heat has the same effect as putting a large electric heater in your CA room, which raises air temperature, which in turn reduces the humidity of the air. You can experience this same effect when you heat your house in the wintertime. The cold outside air, which is relatively humid, leaks into your house and is heated by your heating system. The humidity in a heated house can be very low unless humidifiers are used in the house.
The effect of reduced fan speed on humidity is reversed during pull-down. That is, humidity is lower at reduced fan speed until the fruit and room heat reduces to approximately 1/3 of the total room load including fan heat. The reason for this is that the airflow must be high enough to handle the fruit and room heat without a large temperature rise. As the heat of the fruit and room decreases, then the air volume can be reduced in proportion to the decreasing heat load.
Figures 3 and 4 show why room humidity is reduced if fan speed is lowered too soon after pull-down. Figure 3 shows a temperature profile of the air passing through a CA room when fruit and room load is still high, such as September. Field heat has been removed, but heat of respiration is still high. On the left side of this graph, return air (or "set point") temperature is shown to be 31 ºF.
The next point to the right on the graph shows the air "leaving coil", that is, the evaporator finned coil bundle. At 100% fan speed the air leaves the coil at 30.25 ºF, but at 50% fan speed the air leaves at 30.05 ºF. The refrigerant temperature required in the evaporator is 29 ºF at 100% fan speed, but is 28.4 ºF at 50% fan speed. This reduced refrigerant temperature causes a reduction in humidity leaving the coil as shown in Figure 4. As the air passes through the fan, it is heated ("leaving fan"), and then as the air passes through the room the ceiling, walls, floor, and the heat of respiration of the fruit heat it. I have shown this air temperature rise in segments (1/4, 1/2, 3/4 and Leave) as the air passes through the room. You can see that the air temperature rise is greater at the lower air volume (air volume is proportional to fan speed). At 100% fan speed, air temperature rises approximately 0.75 ºF, versus a rise of nearly 1.1 ºF at 50% fan speed.
Figure 4 shows that room humidity decreases by approximately 1.5% due to reducing fan speed to 50% in September. At this lower humidity, fruit shrinkage will be increased by prematurely slowing down your fans this soon after loading.
My greatest concern with VSD evaporator fan operation is that there seems to be little evidence that the air circulation in a CA room is adequate when the evaporator fan speed is reduced.
Little or no fruit core temperature data is available at reduced fan speeds.
I have talked to a number of people who have experience with fan speed reduction in CA rooms. So far no one has presented any evidence that the airflow is adequate to maintain fruit core temperatures throughout the CA room, when the fan speed is reduced to 50%, or even as low as 40% in some cases.
Air "throw" for an evaporator fan, that is, the amount of momentum that the air has leaving the fans, is directly proportional to the speed of the fan and the diameter of the fan. "Tip" speed, which is the speed at the outer diameter of the fan blade, determines air "throw". As the fan speed decreases, the air momentum is reduced, it will not travel as far, and its ability to overcome backpressure is greatly reduced.
Some operators have told me that when the fan speed is reduced to 50% or 40%, they can feel no air at the far end of the room - the end of the room furthest from the evaporators. I have also been told that core temperatures in some rooms have risen at these low fan speeds. I may be old fashioned, but I still believe that some airflow is required across the fruit, and steady core temperatures are required, in order to maintain good fruit quality.
Modern CA conditions and atmospheres have been greatly improved over the years, but the fruit still does respire and that heat of respiration has to be removed in order to maintain good fruit quality. In my 24 years in the business, I have seen many cases where poor storage design or practices has resulted in "dead air" spaces in the CA rooms, which invariably results in greatly reduced fruit quality.
I strongly recommend that you measure airflow in your CA rooms, or at least a "typical" CA room which represents your room design and bin stacking practices, with the room full and before you seal the room up and begin CA generation. I have a convenient, inexpensive air flow measurement device that I use in my airflow studies. I would be pleased to advise you where to get one of these. Ideally, airflow should be measured at your lowest expected fan speed, and at the far end of the room (a velocity profile) and at the return air end of the room in the far corners.
If possible, measure some core temperatures in the bins at the "far end" and any potential dead corners.
I also strongly recommend that VSD speed reduction should not be used unless "long throw" nozzles are used on the evaporator fans. These nozzles greatly increase the airflow at the far end of the room and are a must in tight stacking situations. Also, please observe the proper bin spacing from the front and back walls. If inadequate space is allowed at these walls, even 100% fanspeed and nozzles, (etc.) aren't enough to get adequate airflow through the bins in your CA room.
Jack W. Staples
3505 Fruitvale Blv'd., Yakima WA 98902
14th Annual Postharvest Conference, Yakima, Washington
March 10-11, 1998