INTRODUCTION
The controlled atmosphere (CA) storage industry has utilized computers for improved control of the storage environment for about ten years. Initially, computers were primarily used for monitoring the levels of oxygen and carbon dioxide in the CA rooms, and controlling vent fans when the oxygen levels drop below desired setpoints. This new technology saved a lot of time in taking manual readings, and did a much better job of maintaining the oxygen level. Since the introduction of computer based monitoring and control of storage facilities there has been an extensive progression of techniques designed to enhance the postharvest operation.
Direct operation of refrigeration equipment has vastly improved temperature control, and maintenance is reduced through decreased runtime and better diagnostic information.
Nitrogen injection to control high oxygen and carbon dioxide levels has facilitated precise atmosphere control. Monitoring and control of on-site nitrogen generation equipment can also be performed by the computer system.
Product shrinkage and shrivel is a serious problem for some varieties that can be minimized by addition of humidity monitoring and control equipment to the system.
Energy management is a significant part of CA storage control systems. Techniques such as fan cycling, variable speed fans, demand defrost, and demand refrigeration not only reduce electrical costs by as much as 50% but also contribute to enhanced product quality. Engine room optimization has become quite refined to provide further operational savings.
The computer control system offers a significant advantage to the storage operation by providing many data management functions such as report generation and historical records needed for international sales. This information is also useful for management decisions, quality analysis, and year to year comparison of operational data.
In the event of an abnormal condition which could be detrimental to product quality the computer system will activate alarms to alert operations personnel. High or low temperature or oxygen level, and high ammonia level are critical conditions that are reported by telephone dialer in order to obtain corrective action before damage occurs.
CA storage buildings are generally unmanned - especially at night or on weekends and holidays. Remote monitoring and control via modem provides a significant advantage in maintaining optimum environmental parameters during off hours or when operating a large number of storage's not located in close proximity.
SYSTEM COMPONENTS
A computer based control system consists of an integrated package of hardware and software components. A standard desktop computer is generally used to provide control functionality and operator interface. A modem is necessary for remote monitoring and control via telephone line. In order for the computer to communicate with sensors and control devices a serial adapter board is installed in the computer to transmit and receive signals from the field control panels.
Hardware
Control panels incorporate analog to digital input/output (I/O) interface hardware that transmits and receives signals from the computer. This hardware converts real world inputs (EG: voltage or current) to a digital signal which the computer can understand. Conversely, a digital signal from the computer can be converted to a real world output by means of the I/O interface hardware. Examples of analog inputs to the computer are: temperature, oxygen level, and relative humidity. Digital outputs from the computer could be a voltage signal that energizes a fan, a valve, or a motor. Control panels are designed to be located near the field devices that are connected to it in order to keep wiring lengths at a minimum. A single data cable that loops to all the control panels is utilized for the transmission of digital signals between the computer and the I/O hardware.
Software
A computer requires software in order to be a useful tool. First, an operating system such as MS-DOS, Windows, OS/2, or Unix is installed to allow basic commands to be utilized. Next, a programming language is necessary to formulate the desired functionality. This programming can be performed with a computer language such as MicroSoft C, or a higher level function block package that provides simplified programming tools.
A graphical interface software package provides basic drawing tools used to develop displays for man/machine interface. These screens utilize digital readings, dynamic objects, trend graphs, simulated buttons, etc. to display information to the operator. Data presentation and management can also be accomplished through printed reports.
Field Devices
CA devices that are typically monitored or controlled by the system are oxygen, carbon dioxide, and ammonia analyzers, power vent fans or nitrogen solenoids, nitrogen generator, and carbon dioxide scrubber.
Refrigeration equipment monitored or controlled by the computer are typically temperature probes, evaporator fans, liquid ammonia solenoid valves, BPR's, defrost valves/pumps, compressors, and condensers.
SYSTEM FUNCTIONALITY
Computer control systems provide continuous monitoring and control of the CA storage environment such that optimum conditions are always maintained. The temperature is controlled with ±.2 F of the desired setpoint by precise modulation of the BPR. The average product temperature is the controlled parameter rather than the air temperature at the evaporator. In addition, good control strategies minimize temperature overshoot after defrost cycles and fan cycle periods. The air temperature drop (coil TD) at the evaporator is measured to provide an indication of refrigeration load, and the BPR can be controlled such that the coil TD does not exceed a maximum value. This feature helps reduce dehydration of the product and extends the time between defrost cycles.
Coil performance degradation is readily detectable by measuring the difference between the return and discharge air. When the zone temperature is above setpoint, and the system is attempting to increase evaporator capacity, if the coil air temperature differential remains below a nominal value for a set time period then the need for defrosting is indicated. CA facilities that implement this strategy often change the defrost interval from one or two defrost cycles per day, to a defrost cycle every week or two. The advantage of demand defrosting is greater temperature stability and reduced heat load in the storage. This can also reduce moisture loss from the product.
Sealed CA rooms are prone to large pressure swings after defrost cycles or fan cycle "off cycles". It is important to initially start only one or two fans for a few minutes when bringing refrigeration back on. After warm air around the coil has been mixed with colder air in the rest of the room, it is then advisable to start the rest of the fans and let them run for another couple of minutes before enabling the back pressure regulator and liquid valve. By the time refrigeration is re-activated, the air temperature has stabilized near the setpoint and the BPR will not over-react to a false load created by the defrost cycle. Consequently, it is good practice to leave the BPR where it was when the defrost cycle began so that refrigeration capacity will match the steady state load for the zone.
Evaporator Fans
When evaporator fans are operated full time after the product cool-down period they produce approximately 70% of the refrigeration load; the balance consists of product respiration and the building envelope loads. Literally all of the energy consumed by the fan motors is converted into heat which the refrigeration system must remove. A typical 2,000 bin CA apple storage room will have (4) three horsepower fan motors which use approximately 10 kilowatts of power. This adds as much heat to the room as would (4) ten foot long electric baseboard heaters. It's no wonder that refrigeration compressors carry a significant load even when ambient temperatures are below freezing!
In addition to the negative impact that full capacity fan operation has on energy efficiency, the product quality can also be adversely affected. Research has shown that the excess evaporator capacity tends to overdrive the temperature which results in less stable product temperature than even intermittent coil operation does. The high air velocity that the fans produce seriously affect the rate of moisture loss, or transpiration, from the product. This equates to less product left to sell (as much as 2-5% weight loss) and lower quality due to shrivel, etc.
Intermittent operation of evaporators, known as fan cycling, has been widely used to increase energy efficiency and improve product quality in CA storage's. While the air temperature near the ceiling of a CA room can rise a couple degrees within an hour or two after shutting down the evaporator, the product temperature changes very slowly due to the thermal mass and the small heat load. It is important to place the temperature probes used for controlling the room temperature in the vicinity of the product so that the product temperature drives the system rather than air temperature. Ideally, the probes should be "buried" under a thin layer of the product to obtain a representative temperature. If this is not practical because of the requirement to have personnel place the probes when loading the rooms, and remove them when taking the product back out, it is possible to have the probes hang down in the corners of the room, halfway between the ceiling and the floor, such that they will be very near the product that is stored.
The benefits realized by fan cycling are a drastic reduction in energy usage and improved product quality. Defrost intervals can be extended because less frost accumulates on the coils.
Variable speed evaporator fan control is an alternative to fan cycling which offers the same benefits and increased advantages. This technique utilizes AC variable speed drive units to operate the fans over a typical speed range of 50 to 100% of full speed. The speed is varied proportionally to the back pressure regulator (BPR) position.
The advantages offered by variable speed fan control are as follows: maximum fan energy savings, full time operation of evaporators, uniform temperatures, reduced wind scrubbing of product, and enhanced flexibility with fan/BPR temperature control relationship.
CA Monitoring and Control
CA facilities have some unique equipment for monitoring and controlling the storage atmosphere. Oxygen and carbon dioxide analyzers are connected to a sample draw system that sequentially pulls the atmosphere from each zone and measures the level of these gases. In this manner, storage environment readings are frequently being updated and corrective action taken.
Nitrogen is initially used to flush the oxygen out of the sealed rooms, and later to purge the carbon dioxide that is given off as a product of respiration of the stored commodity. Once the oxygen levels are near setpoint, the nitrogen is primarily used to control the continual buildup of carbon dioxide. The electrically operated nitrogen injection valve is energized for a time interval proportional to the deviation from desired setpoint. The greater the deviation, the longer the valve remains open. Each time the room is sampled the nitrogen injection period is re-calculated until the oxygen and carbon dioxide levels do not exceed the entered setpoints.
The computer control system energizes a vent fan when the oxygen level in a storage room falls below setpoint. The vent fan blows air into the room for a time period determined by the deviation from setpoint, similar to the nitrogen valve control scheme.
Carbon dioxide removal may also be achieved by controlling molecular sieve scrubbers and electrically operated valves as required.
The oxygen level is maintained with ± .1% of desired setpoint using these techniques. This permits operating at a very low level of oxygen in order to maintain the highest product quality.
The sampling system also monitors the atmosphere for an ammonia leak in the storage rooms. If a high ammonia level is detected, alarms are activated and the computer system initiates a defrost cycle in the affected zone in order to absorb the leaking refrigerant into the defrost water. In addition, the BPR is fully opened to facilitate pumping the liquid out of the coil.
CONCLUSION
There are many ways that computer control systems can improve control of storage regimes. The net result of precise temperature and oxygen level control is better product quality that is essential in today's marketplace. The advantage of better data acquisition is essential for some foreign markets, and useful for record keeping and management decisions. Significant operational cost reductions are realized from more efficient energy utilization, and reduced labor and maintenance expenses. Finally, the control system can minimize the risk of casualty loss by continuous monitoring of critical parameters and providing automatic response and alarm functions.
