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.
