Moisture in Solids

					TIM 024 07.97

 Moisture in Solids
 Moisture Measurement for Control when Drying Solid Materials in Solids
Many industrial drying processes call for moisture analysis        Baking ovens, as well as many types of drying ovens in
as an indication of product quality or process control. An         general, use hot air to dry a product as it passes through,
example can be found in the food industry where product            typically on a conveyor. The flow diagram for such a
specifications often include a tolerance for moisture content.     dryer/oven system is quite simple.
The moisture content of most breads, chips, cookies, and
                                                                   Heat is used in these systems to initiate the chemistry of the
other baked goods greatly affects the quality of the product.
                                                                   baking process as well as the drying function. The warmer
In applications where materials are being loaded into
                                                                   air is, the more capacity it has to hold moisture, and
railroad cars or aboard cargo ships the moisture content can
                                                                   therefore, dry a certain product. In such a system the product
make a significant difference in the density of the material.
                                                                   enters the oven with a certain known moisture content. The
In these cases the moisture measurement would give an
                                                                   hot air flow comes in contact with the product and moisture
indication of the weight of a given load or the storage space
                                                                   is removed. The flow of hot air allows the moisture to be
required to hold it. In processes where solid materials are
                                                                   exhausted out the of the oven ductwork while the product
mixed and blended the moisture content of the ingredients
                                                                   exits on the conveyor. The percent moisture left in the
can effect the consistency, flow characteristics, and quality
                                                                   product depends on how much was removed by the oven.
of the end product.
                                                                   The amount of moisture removed by the oven depends on
Measuring the moisture in solid material applications can be       four primary variables. The variables are the amount of
an expensive endeavor for the customer. If a customer wants        moisture initially in the product, the temperature of the air,
to know the moisture in a solid, expressed as a percentage         the flow rate of the air, and the amount of time the product
they will often have to turn to near infra-red sensor              spends in the oven. The basic process control of such a
technology. This measurement technique uses a beam of              system involves the proper adjustment of the last three
near infra-red light. The light reflected off the solid material   variables which will, in turn, determine the final moisture
is analyzed by the measurement device to determine how             content in the product.
much of the near infra-red energy was absorbed. The surface
                                                                   General Eastern can offer customers a way to achieve the
moisture content of the solid can be determined by the
                                                                   process control without paying for the high cost of an near
change in the amount of reflected near infra-red energy.
                                                                   infra-red system. A General Eastern MDR moisture probe
These systems can be difficult to fine tune for a given solid,
                                                                   could monitor the moisture content of the air before it enters
and they are usually expensive. Typical systems run between
                                                                   the oven. A second MDR probe could monitor the moisture
$20,000 and $30,000 per point of measurement.
                                                                   content of the air as it is exhausted from the oven. A
The General Eastern takes many phone calls from potential          HygroTwin 2850 analyzer could then show the difference in
customers asking if our moisture analyzers can determine           moisture between the inlet and outlet. The moisture content
the percent moisture content in some type of solid material.       of the exhaust will inevitably be higher because moisture is
The answer to that question is, no, we cannot determine the        added to the air stream from the product while inside the
percent moisture content in solids with our moisture               oven. If this increase in moisture between is monitored, and
analyzers. This not the end of the story, however.                 if the initial moisture content is known (from a recipe), the
                                                                   final moisture content of the product can be controlled.
For example, baking not only involves the chemical action
that takes place within a baked product, it also involves the      The HygroTwin brochure describes several differential units
removal of moisture. Let's look at a customer who bakes            of measure included in the 2850 series analyzers for this
cookies and wants to know the percent moisture in his              type of application. By monitoring the moisture differential
cookies as they exit his baking oven. They will have a             across the dryer or oven and periodically testing the percent
specification for the maximum and minimum amount of                moisture content of the product using an inexpensive spot-
moisture he can tolerate in the cookies he will sell. While        check type solids moisture analyzer the customer can
this customer needs to maintain a certain moisture content in      determine the proper control settings. The customer may
the cookie, the means of doing this is by properly controlling     find that for a given conveyor speed, air flow rate, and
the baking oven. If the baking oven has some type of control       temperature the moisture differential must be within a
scheme to govern the rate at which the cookies will be dried,      certain range in order to produce the desired moisture
the end result can be achieved. This is where the General          content in the final product.
Eastern moisture analyzers can assist in fine tuning the
process control.

The first thing to do when evaluating a moisture in solids        however, process control is the goal, that is what General
application is determine if a drying process is involved.         Eastern has specialized in for over forty years. Finally, the
Next, determine if a continuous percent moisture indication       application may have distinct zones with exhaust ducts for
is required or if the goal is simply to achieve the proper        the air. Often these zones will have different rates of drying
process control. If it is necessary to know the actual percent    the product. Moisture monitoring in all the ducts will allow
moisture content in the solid on a continuous basis the           more precise control of the overall drying process.
General Eastern moisture analyzers are not the answer. If,

                                                General Eastern Instruments, Inc
                                            20 Commerce Way Woburn, MA 01801                                                  2
                                             Tel 781-938-7070 Fax 781-938-1071