WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
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ITG SUBJECT: STOCK ROTOMAT
The ROTOMAT, a discontinuous or batch type, end-over-end agitating retort was developed by Hermann Stock Maschinenfabrik, GmbH; New Munster, West Germany in close collaboration with the Hamburg firm Mittelhauser-Watter. These units are distributed in the United States by Stock America, Inc.; Milwaukee, WI. The basic concept behind development of the sterilizer was to avoid burning the product around the inner sides of the container and to speed up heat transfer to the coldest point by rotating the baskets holding the containers in a high-pressure horizontal retort, submerged in a sterilizing medium of water, pre-heated in a storage tank and circulated in the system utilizing a high-pressure centrifugal pump.
Some of the benefits claimed for the ROTOMAT are:
It saves up to 80% of the normal retorting time in a still retort through a combination of:
a) High temperature - short time. b) End-over-end rotation. c) Continuous forced circulation of high temperature water in the working drum.
- Water and energy consumption is greatly reduced because the water used as a sterilizing medium is pumped back to the unit's storage drum and then reused.
- The unit operates completely automatically, according to one of several modes, among them, electric switch, punch card and microprocessor or computer control.
Stock manufactures a variety of rotary over-pressure retorts for the thermal processing of foods, of which the ROTOMAT is one version. In addition, static (still) versions of these retorts are available. The rotary models include the ROTOMAT and JUMBOMAT (full water immersion as a heating medium) and the ROTOVAP (water spray or cascade as a heating medium). The AUTOMAT is the still version of the ROTOMAT, the AUTOVAP is the still version of the ROTOVAP. Capacities in these units range from 1 to 5 cages or baskets. Container capacity in the JUMBOMAT is three times that of the standard four cage model due to the size of the baskets. The basic unit in the full-immersion models, however, is organized as a two-drum (over-under) system with a top storage vessel or drum used to pre-heat water used as the sterilizing medium; and a lower or working (sterilizing) drum where the baskets are agitated, and the sterilizing water circulated by being drawn out through a suction manifold in the bottom of the drum, pumped through a heat exchange system (direct steam injection), then discharged through a distribution manifold located along the top of the drum. The organization of the ROTOMAT is detailed in Figure #1 (image size 10KB) as follows:
Water is preheated in the top storage vessel (1), generally to a temperature not lower than 260-275 F. In some cases, depending upon the scheduled process, the temperature can be in the range of 290-300 F. A "rule of thumb" is to heat the water in the storage drum to a temperature of at least 10 F higher than processing temperature. At the beginning of the sterilizing cycle, water is dropped to the lower processing or working drum (2). A counter-pressure which was generated during the heating of the upper drum, assists in moving the water to the lower drum. At the time the processing water is dropped to the working drum, the baskets or cages begin to be rotated end-over-end by a variable speed drive motor which turns the rotary device or reel (3). The reel can be rotated in either direction or can be oscillated. Rotation can be programmed within a range of 6-46 RPM.
The process of moving the preheated sterilizing water to the working drum involves dropping it from the upper storage drum, to the positive displacement pump located at the rear of the unit (4). The pump moves the water through the circulation line (5) where steam is injected (6) to insure maintenance of the programmed temperature; then to a distribution manifold (7) located along the top of the working drum. Water flows down over the crates and is drawn out through a suction manifold (8) located along the bottom of the working drum and sent back through the circulation line again. This completes the circulation system for the sterilizing water.
At the beginning of the cooling cycle, the hot sterilizing water is pumped back up to the storage drum where it is re-heated to the proper drop temperature for the next cycle. At the same time the cooling water pump (9) starts and pumps cold water into the circulation line. The cold water mixes with the hot water during the initial stages of the cooling cycle. The temperature of the sterilizing water is maintained at approximately 212-215 F by the time it is pumped back to the storage drum. Thus, re-heating the upper drum takes less time after the initial sterilizing cycle. During the cooling cycle, and the subsequent draining phase, the reel continues to rotate.
All automatic valves used in the ROTOMAT are electro-activated ball valves. The steam control valves for heating the upper drum, and maintaining the proper temperature in the working drum, have five second actuators. A tacho generator is used to control the reel speed with a higher voltage resulting in higher RPM. The ratio is approximately 1.1 to 1.2. For example, 15 volt AC represents approximately 12 RPM. The back plate of the rotary device is flexible, so even if the two front rollers on which the reel rests wear out, the reel will still turn. The guide (transfer) rollers along the side of the reel bear the weight of the cages so they tend to wear out sooner.
Two types of cages are used with the ROTOMAT - one slotted, one perforated. The slotted cage has at least 50% total open space. The newer, perforated cage has approximately 60% open space. Two types of spacer mats have been used, as layer dividers and on top of cages in varying numbers (depending on the size of container being processed) to provide contact between the top locking plate of the reel and the top layer of containers in a cage or basket. The older design consisted of square perforations with approximate 1 1/8" sides, in 18 rows of 20 squares each for a total open area of approximately 61%. The newer design employs square perforations with 3/4" sides in 21 rows of 24 squares each for a total open area of approximately 35%. The newer design also employes "ribs," along the periphery of the perforations, which are channeled on the bottom to provide for proper water circulation.
The ROTOMAT Model RN comes equipped with a 2-track ink strip chart recording device. Recordings are made for temperature and pressure. Reel speed (in RPM) is monitored by a needle-type indicator. The Models RE and and RSE come equipped with a three-pen spark-erosion (the tracing is actually burned into the paper) strip chart recording device. Recordings are made for pressure, RPM, and temperature, (all in the working drum). Two gauges located on the upper portion of the recording device indicate the pressure and temperature in the upper storage drum. Units sold in the U.S. have been modified to have an attached Taylor recording device either in lieu of, or in addition to, the spark-erosion strip chart, which did not always conform to 21 CFR Part 113.40 re: chart temperature graduations. Generally speaking, if one function is controlled (i.e. temperature) a twelve hour Taylor chart is employed. If more than one function (i.e. temperature and pressure), a 24-hour chart is used. The Rotomatronic S, (computer-controlled version) does not routinely come equipped with the spark-erosion strip chart recorder, relying instead on the Taylor recording thermometer. Both the spark-erosion strip chart and Taylor recorders serve only to record, not to control the temperature in the working drum.
Steam control in all models of the ROTOMAT is effected by a PT-100 resistance temperature device (RTD) located either just below the circulation line, or in the circulation line itself, on the electronic switch program model and the card reader model reader, respectively. The Rotomatronic S has the PT-100 located in the right front side (when facing the unit) of the working drum in proximity to a MIG thermometer. The ROTOMAT originally did not come equipped with an MIG; however, it was subsequently added to all models in the U.S. to meet requirements of 21 CFR Part 113.40.
The sequencing of the sterilization and cooling cycles in the ROTOMAT is completely automatic (except for the Model RG) and controlled in one of five different modes (called "models"):
- Model RG - A manual switch-operated system with different colored illuminated buttons (light-buttons) to operate all valves, except those for temperature and counter-pressure control, which are operated automatically. The RG is similar to the RN except there is no recording device. The only RG models sold in the U.S. have been for research only.
- Model RN - An electro-mechanical program control system. The processing program is selected by the manually pre-setting various switches, and runs automatically. Process recording is by a 2-track ink strip chart recorder (see Figure 2 - image size 6KB). Recordings are made for temperature and pressure. No model RN are currently in use in the U.S.
- Model RE - Electronic pre-programmed logic control (PLC). The pre-set instruments consist of a series of dials/selector switches which are manually set on the control panel. The various steps in the process are indicated by various colored illuminated buttons (light-buttons). The process is recorded (temperature, pressure and RPM) by spark-erosion on a metal foil strip chart recorder (see Figure 3 - image size 8KB). It should be noted that in the past, STOCK vendored the models RN & RE as a single control version both employing the electro-mechanical control system and spark-erosion strip chart recorder. If vendored in the U.S., a Taylor circular recording chart for recording temperature would also be supplied to supplement the 3-pen spark erosion recorder. No model RE's are currently in use in the U.S.
- Model RSE - Electronic control by punch card, or the "card-reader" system. The program is established according to a program sheet. Once the program has been written on the sheet, corresponding squares are punched out on the program card for the respective products. Once the card is placed into the proper slot in the control panel for the unit, the holes in the card come in contact with limit switches, closing these switches and causing the machine to advance through a processing cycle from heating the upper drum to drain (see Figure 4 - image size 5KB).
- Rotomatronic - Micro processor or computer controlled system (see Figure 5 - image size 3KB). All steps in the processing cycle are programmed into the computer using function and alpha-numeric keyboards, both employing pressure-sensitive keys. The computer has two basic software packages, consisting of chips. One is a pre-designed system for the unit to sequence through each step in the processing cycle, e.g., Heating the Upper Drum, Sterilization I, Sterilization II, Cool I, etc. This is basically a PROM (programmable read-only memory) which is permanently burned on and cannot be changed. Therefore the sequence of operation itself cannot be changed by anyone at the cannery. The other package is the actual input values for the target temperatures for heating the upper drum, scheduled RPM, process time and temperatures, etc. These values can be changed by accessing the computer through an access code. The programmer is directed via instructions illuminated on a LED screen on the control panel, while the program values are being written. Prior to starting a cycle, the retort operator enters certain values, such as operator number, batch number, product number (which determines the time, temperature, pressure and RPM of the process) and initial temperature. This information is necessary to begin heating the upper drum. During a processing cycle, the LED screen will inform the retort operator what the programmed values and actual values are, as the machine progresses through each step in the cycle. At the end of the cycle, a computer-generated processing record (CGPR) is printed out, listing the times, temperatures, reel speed and other critical factors. In addition, any alarm conditions, such as low temperature noted during the process, are also printed out. Generally no audible alarm is sounded, nor any other visible indication given, should one of these conditions occur. At the end of the cycle, the retort operator has the option of reviewing each step on the LED screen, and having another copy of the CGPR printed. In some instances, the firms may maintain a hand-written processing record to complement the CGPR.
There are currently alarm functions in the software for temperature, pressure and RPM. According to Stock, a new software package has been prepared that will also alarm in the event a low water level is reached, even though this has not been determined to be critical from a thermal processing stand point. There will also be a fifth alarm function which will be left "open" in the package. That is, any individual authorized to program the machine will be able to program a fifth alarm condition of his or her choosing.
In addition, another software change (in chips) has been prepared which will give a printout of temperature, RPM or pressure every 30 seconds, should there be a deviation from scheduled parameters.
SEQUENCE OF OPERATION
After pre-heating the water in the upper drum to the programmed temperature, the basic sequence of operation in the ROTOMAT involves a one or two stage come-up (depending on the model); a hold phase at processing temperature; a two-stage cool and a drain phase. The sequence is as follows (see Figure 6 - image size 12KB):
At the time the sterilizing cycle begins, the connection valve (1) opens and the circulation pump (2) is activated. Water pre-heated to the programmed temperature flows through the connection line to the pump which moves it through the circulation line (3) where steam is injected via the heating valve (4) in decreasing amounts until process temperature is reached. At that time the heating valve functions as a proportional controller, maintaining the programmed holding temperature. At the same time the pump is activated, the reel begins to rotate the cages or baskets and the vent valve (5) opens to remove air from the working drum. This vent valve can remain open from 1/2 to 2 minutes, (this is programmed on the basis of time only, not on water level or any other function) although the normal time frame is approximately one minute. Air is removed from the working drum by the super heated, pressurized water from the storage drum entering atmospheric conditions in the working drum and flashing to steam. This pressurizes the working drum and forces the air out the vent line.
Water is pumped through the circulation line, through the throttle valve (6) to the distribution manifold (7) along the top of the working drum. The distribution manifold has approximate one inch ports opening to the lower drum. Water is drawn out through a suction manifold (also called the "circulation channel") along the bottom of the working drum. The manifold has square, approximate one inch ports, each covered with wire mesh. It takes approximately two to three minutes to fill the lower working drum to the desired level. ROTOMAT in the U.S. are generally equipped with a water level sight gauge. Conversations with Stock America and another processing specialist familiar with the operation of the ROTOMAT indicate that ensuring a water level above the top of the cages is not critical from a heat distribution standpoint, since over-pressure is provided by steam.
On the Models RG (semi-automatic), RN/RE (electric switch and PLC) and RSE (card-reader), the time from the drop of pre-heated water to the circulation pump, until processing temperature is reached, is designated as "Sterilization I." On the Rotomatronic, the come-up phase has been split into "Sterilization I" - from the beginning of the drop of pre-heated water to the close of the vent valve - and "Sterilization II" - from the closing of the vent valve until processing temperature is reached. It should be remarked that a minimum come-up time (CUT) has been designated a critical factor for certain products and this will have been programmed into the machine. This CUT may be 7-11 minutes depending upon the initial temperature stipulated by heat penetration studies. The reason for dividing the CUT into two phases in the Rotomatronic, is that use of a computer allows for more exact control.
The hold phase at processing temperature has been designated "Sterilization II" for the Models RG, RN/RE and RSE; and "Sterilization III" for the Rotomatronic. At the beginning of the hold phase in the Rotomatronic, the operator is alarmed to read - and input into the computer - the MIG and recording thermometer readings and to time the reel speed. Input of these values has no effect on the programmed process time; however, the readings are listed on the CGPR. Reel speed is checked on the Rotomatronic by the operator observing the number of times a light on the control panel, which indicates the baskets are in the horizontal position, blinks on and off in a minute.
Pressure in the working drum (normally in the range of 2.0-3.0 bars or 30-44 PSI) is indicated by a dial gauge on the door in all models. It is also continuously recorded on both the spark-erosion type and Taylor recording devices. Pressure in the working drum has not been declared to be a factor critical to the scheduled process.
At the end of the scheduled process time, the machine sequences into the first cooling phase ("Cool I"), then into the second ("Cool II"). Total cooling time is approximately 20 minutes. The reel continues to turn at the scheduled speed during these two phases. Following Cool II, the machine sequences into the last phase - the drain. The reel speed normally is slowed at this point. Recovery of the sterilizing water in the upper storage drum is completed by the end of Cool I and the connection valve (1) closes at this time.
Proper pressure in the ROTOMAT is achieved by steam. Air is not used unless specifically requested by a processor. In this case, air would be supplied through the "auxiliary line" (15) shown in Figure #6 - image size 12KB). Stock generally prefers to establish and maintain over-pressure with steam rather than air.
During the "Heating Upper Drum" phase, water is heated initially to 212 F at which time steam (10) is programmed to be injected through a 2 1/2 inch line controlled by an automatic valve (11) to provide an over pressure of 2-2.5 bars (29.5-37.0 PSI). If the pressure rises above the programmed value, it is released via the small vent valve (12). Once the superheated water is dropped through the connection line, the connection valve (1) remains open until the beginning of Cool II. This allows steam and non-condensable gases accumulating in the head space of the working drum to rise through the connection line and accumulate in the head space of the storage drum, from which they would be released via the small vent valve. This provides for the maintenance of a proper pressure balance between the upper storage and lower working drums during the sterilization phases.
A particular pressure is also programmed for the cooling phases. This is controlled via the pressure drop regulating valve (13) and the vent and pressure drop regulation valve (14). If the pressure gets too high during the cooling phase, steam is vented out through these valves and more water is admitted. If the pressure gets too low, the steam is injected to maintain the product over-pressure.
HEAT DISTRIBUTION AND HEAT PENETRATION
Adequate heat distribution in the ROTOMAT is dependent upon such factors as the proper functioning of the centrifugal pump, rotation of the reel and the number of cages in the retort. The number of cages also effects the CUT to the holding (processing) temperature as does the initial temperature, with IT having, perhaps, the greater effect. In addition, the open area provided by divider plates or spacers, as well as the cages themselves, can have an effect on adequate heat distribution. Any change to a more restrictive-to-flow design in any of the above must be validated by new heat distribution studies.
The lack of rotation has been shown to cause a temperature stratification in some instances with cooler temperatures being in the lower part of the working drum.
With respect to the proper function of the pump, all ROTOMAT models have visual indicators which signal if the pump is not working properly. That is, whether or not it is on. There are currently no provisions for monitoring the flow rate (e.g., by a flow meter) which would indicate if the pump is delivering the desired flow. According to Stock, the circulation pump has a maximum capacity of approximately 400 gallons per minute. During Sterilization I, the pumping rate is variable. During Sterilization II (for the RG, RN/RE and RSE) and Sterilization III (for the Rotomatronic), the pumping rate is approximately 260 gallons per minute. Stock has stated that no significant effect has been noticed on heat distribution down to a rate of 100 gallons per minute. Below that, it could be questionable although a determination has been made that the rate would probably have to get down to practically zero to have a noticeable effect, providing the reel is rotated.
However, according to Stock, any case of the circulating pump going completely down, would have to be treated as a process deviation. Any evaluation for public health significance would involve reproducing such a condition while conducting a heat distribution study.
With respect to the wearing out of the pump impellers, and any possible effect this might have on heat distribution, Stock has seen little effect. Data from 6 to 7 year old studies where pump impellers were quite worn (glass pack operation) indicated no problem.
Because of the various factors which can affect heat distribution and heat penetration in the ROTOMAT, an agitating process should never be adopted by a processor until comprehensive heat distribution and heat penetration studies have established the point of least sterilizing valve in the container and in a specific location in the basket. With respect to location of the "cold spot" or point of least lethality in the container, variations in fill, viscosity, and solid-to-liquid ratios can have significant effects. With respect to the location of a particular can in the basket, consideration must be given to the variable rotational path of the container, depending upon whatever it is in the center of the basket or adjacent to the basket wall. In many instances, the most critical aspects of heat penetration are minimum head space and rotation of the reel. Even a slight (1/16") change in head space can have a significant effect on heat penetration, when other factors are held constant.
Cathers, H., December 22, 1986. Personal Communication.
Cathers, H., 1985. Remarks at Seminar on Operation of the ROTOMAT. FDA/Center for Food Safety and Applied Nutrition, Washington, DC.
Eisner, M., 1972. Introduction into the Technique and Technology of Rotary Sterilization Verlay Gunter Hempel - Bramschmsig, Federal Republic of Germany.
Lopez, A., 1981. A Complete Course in Canning. Book I - Basic Information on Canning. 11th Edition. The Canning Trade, Baltimore.
Stock Bulletin, 1986. Stock AUTOMAT/ROTOMAT/JUMBOMAT Full inversion overpressure retorts for static or rotary sterilization PN 7/01 86/500 d/GB/-M/ Payasus Werbeagentur Hamburg.
Stock Bulletin, 1985. Stock AUTOVAP/ROTOMAT. Spray Circulation overpressure retorts with direct or indirect heating and cooling. For static or rotary sterilization PNG/10.85/5000D/GB/-M/Peyasus Werbeagentur Hamburg.
Stock Bulletin, undated. Sequence of Operation Stock America, Inc. Milwaukee, Wisconsin.
Tosca, G., August 21, 1986. Personal Communication.
Illustrations used with permission of Stock America, Inc.
Heating Storage Drum