Moisture in Solids

Moisture in Solids

Moisture Measurement for Control when Drying Solid Materials in Solids

TIM 024 07.97

Many industrial drying processes call for moisture analysis as an indication of product quality or process control. An example can be found in the food industry where product specifications often include a tolerance for moisture content. The moisture content of most breads, chips, cookies, and other baked goods greatly affects the quality of the product. In applications where materials are being loaded into railroad cars or aboard cargo ships the moisture content can make a significant difference in the density of the material. In these cases the moisture measurement would give an indication of the weight of a given load or the storage space required to hold it. In processes where solid materials are mixed and blended the moisture content of the ingredients can effect the consistency, flow characteristics, and quality of the end product.

Measuring the moisture in solid material applications can be an expensive endeavor for the customer. If a customer wants to know the moisture in a solid, expressed as a percentage they will often have to turn to near infra-red sensor technology. This measurement technique uses a beam of near infra-red light. The light reflected off the solid material is analyzed by the measurement device to determine how much of the near infra-red energy was absorbed. The surface moisture content of the solid can be determined by the change in the amount of reflected near infra-red energy. These systems can be difficult to fine tune for a given solid, and they are usually expensive. Typical systems run between $20,000 and $30,000 per point of measurement.

The General Eastern takes many phone calls from potential customers asking if our moisture analyzers can determine the percent moisture content in some type of solid material. The answer to that question is, no, we cannot determine the percent moisture content in solids with our moisture analyzers. This not the end of the story, however.

For example, baking not only involves the chemical action that takes place within a baked product, it also involves the removal of moisture. Let's look at a customer who bakes cookies and wants to know the percent moisture in his cookies as they exit his baking oven. They will have a specification for the maximum and minimum amount of moisture he can tolerate in the cookies he will sell. While this customer needs to maintain a certain moisture content in the cookie, the means of doing this is by properly controlling the baking oven. If the baking oven has some type of control scheme to govern the rate at which the cookies will be dried, the end result can be achieved. This is where the General Eastern moisture analyzers can assist in fine tuning the process control.

Baking ovens, as well as many types of drying ovens in general, use hot air to dry a product as it passes through, typically on a conveyor. The flow diagram for such a dryer/oven system is quite simple.

Heat is used in these systems to initiate the chemistry of the baking process as well as the drying function. The warmer air is, the more capacity it has to hold moisture, and therefore, dry a certain product. In such a system the product enters the oven with a certain known moisture content. The hot air flow comes in contact with the product and moisture is removed. The flow of hot air allows the moisture to be exhausted out the of the oven ductwork while the product exits on the conveyor. The percent moisture left in the product depends on how much was removed by the oven. The amount of moisture removed by the oven depends on four primary variables. The variables are the amount of moisture initially in the product, the temperature of the air, the flow rate of the air, and the amount of time the product spends in the oven. The basic process control of such a system involves the proper adjustment of the last three variables which will, in turn, determine the final moisture content in the product.

General Eastern can offer customers a way to achieve the process control without paying for the high cost of an near infra-red system. A General Eastern MDR moisture probe could monitor the moisture content of the air before it enters the oven. A second MDR probe could monitor the moisture content of the air as it is exhausted from the oven. A HygroTwin 2850 analyzer could then show the difference in moisture between the inlet and outlet. The moisture content of the exhaust will inevitably be higher because moisture is added to the air stream from the product while inside the oven. If this increase in moisture between is monitored, and if the initial moisture content is known (from a recipe), the final moisture content of the product can be controlled.

The HygroTwin brochure describes several differential units of measure included in the 2850 series analyzers for this type of application. By monitoring the moisture differential across the dryer or oven and periodically testing the percent moisture content of the product using an inexpensive spot-check type solids moisture analyzer the customer can determine the proper control settings. The customer may find that for a given conveyor speed, air flow rate, and temperature the moisture differential must be within a certain range in order to produce the desired moisture content in the final product.

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