SUBJECT: STEAM DISTRIBUTION FOR RETORT VENTING IN FOOD CANNERIES
Following is a brief description of some points to consider when evaluating
the adequacy of the steam supply for venting the retorts in food canneries
using batch-type retorts for cooking. Observation of any one or more of
three critical points: low retort header pressure, undersized retort header,
or use of a vent schedule shorter than recommended for the installed steam
inlet size, provides ample reason for concern and collection of detailed
data from several successive retort cycles. Header Pressure: After inspecting
the boiler-room or power plant, it is desirable to follow the main steam
header into the cannery. Steam is required, in a typical plant, at two
distinctly different pressures; at approximately 90 psi, and at approximately
15 psi. Generally, steam at 90 psi is required only for venting the retorts,
\1\, \2\, \3\, while steam at approximately 15 psi is required for almost
all other equipment, such as washers, caustic peelers, blanchers, exhaust
boxes, vacuum sealers, syrup tanks, hot water heaters, space heaters, and
for the cooking portion of the retort cycle. \3\ A well designed plant
will have an 80 to 100 psi header branch carrying steam directly from the
main header to the retorts, and another branch header on which is installed
a pressure reducing valve and a safety relief valve, carrying 15 to 30
psi steam to all other steam consuming equipment. Header Pipe Size The
authorities cited above (\1\, \2\, \3\) specify the following minimal retort
header pipe sizes:
number of Retorts | Vertical & Horizontal | Horizontal Retorts
|
Venting Simultaneously | Retorts Less Than 15'Long | Over 15' Long
-
|
1 | 2 inches | 2 - 2-1/2 inches
|
2 | 2-1/2 inches | 3 - 3-1/2 inches
|
3 | 3 inches | 3-1/2 - 4 inches
|
4 | 3-1/2 inches | 4 - 5 inches |
Two inspection problems are anticipated; the steam header will usually
be covered with insulation, and these nominal pipe sizes do not correspond
to either the inside or outside actual diameters of the pipe. Close examination
of the length of the header will almost certainly reveal a location where
the pipe itself can be seen and measured. The following table gives actual
outside diameters corresponding to nominal pipe diameters. All ordinary
steel pipe used for water or steam service in food plants will be either
schedule 40 or schedule 80 (wall thickness) and either schedule is made
to the same outside diameter.
Nominal Pipe Diameter | Actual Outside Diameter |
Inches | Inches |
1 | 1.315 |
1-1/4 | 1.660 |
1-1/2 | 1.900 |
2 | 2.375 |
2-1/2 | 2.875 |
3 | 3.500 |
3-1/2 | 4.000 |
4 | 4.500 |
5 | 5.563 |
Venting Schedules The venting schedules recommended by Lopez \3\ and
by Continental Can \2\ depend upon the size of the steam inlet into each
individual retort, from a minimum of 1 inch up to a maximum of 2 inches.
NCA \1\ recommends only a venting schedule for a retort equipped with a
minimum 1 inch steam inlet. Presumably, NCA recognizes that larger steam
inlets may be used, but is unwilling to recommend shorter venting schedules
without experimentation on the particular retort installation. Reference
to these authorities should be made for detailed venting schedules. At
the conclusion of the venting operation the steam flow entering the retort
is throttled down to a pressure (typically 10-15 psi) corresponding to
the desired cooking temperature, and the cooking schedule is begun. Maintenance
of this temperature by controlling the pressure (directly or indirectly)
is extremely important throughout the cook cycle. This is done in a variety
of ways and will be discussed in a subsequent ITG. \1\Processes for Low-Acid
Canned Foods in Metal Containers, Bulletin 26-L, National Canners Association,
Washington, D. C. 20036. \2\Retorts for Canning, Bock, J.H., Continental
Can Co., Inc., Chicago, Illinois. \3\A Complete Course in Canning, Lopez,
Anthony; The Canning Trade, Inc., 2619 Maryland Avenue, Baltimore, Maryland
21218.
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