CSD_PSY 101 - Basic Drying Psychrometrics
Joe Jacobsen
Custom Seed Drying Solutions - ?? | President, Founder and CEO at Jacobsen Holz Corporation. Helping Seed Companies of All Sizes Realize Their True Drying Potential At Scale ??
Overview
The basics of drying are generally understood by most people.
If you leave a moist object, such as a sponge, on a surface in open air it will eventually dry out. If you add a little breeze, it will dry out quicker. If the sun is shining on it as well, it will dry out much quicker.
While it can probably be said that everybody understands and makes use of these "basic" drying principles in everyday life, not many understand the "science" behind these principles.
The purpose of this article is to convey to the reader a basic understanding of the science that governs the seed drying process. This will provide a better working knowledge of Drying Psychrometrics and how the different drying parameters impact ear corn drying.
Basic Drying Psychrometrics
While there are different types of drying processes, the most common is termed, "Convective Drying". Convective Drying employs the phenomenon called Evaporative Cooling to evaporate water molecules present at the exterior of the ear corn's kernels and cob.
This is the drying process implemented in ear corn drying.
In order to bring about Evaporative Cooling in ear corn drying, the heated air must possess sufficient energy to "force" the water molecules to migrate towards the exterior of the kernels and cob and then supply energy for the water to transition into a vapor state or to evaporate.
Based on the energy available in the air, the kernels and cob of the earn corn are seeking to reach a state known as Moisture Equilibrium with the air. This makes it necessary for the drying hear source to provide sufficient energy to the air so that the desired moisture level can be reached in the drying process.
The water vapor that provides the "force" that causes the migration of moisture to the exterior of the ear corn is Vapor Pressure.
When Vapor Pressure inside the ear corn's kernels and cob is greater than the Vapor Pressure of the surrounding air, the water molecules move toward lower Vapor Pressure, which is the water vapor in the air volume surrounding the ear. When the Vapor Pressure of the air volume surrounding the ear is equal to the Vapor Pressure in the seed, we reach "Moisture Equilibrium". The difference between the Vapor Pressure of the surrounding air volume and the of the ear corn is called the Vapor Pressure Deficit, or (VPD).
As the water molecules reach the exterior of the ear corn surfaces, energy I the air causes the molecules to change to a vapor state, or evaporate.
The net result of this process is the "expenditure" of the energy contained in the air. The net effect of this is the reduction of the VPD and the rat of drying. If the air surrounding the earn corn is stagnant, the VPD eventually reaches zero (which constitutes "moisture equilibrium") and drying stops.
Air flow alleviates the issue of VPD and Dry Rate reduction by transporting the moisture laden air that surrounds the ear corn elsewhere; usually to the atmosphere. Air flow serves to maintain a somewhat "steady" VPD and Dry Rate.
This is an overview of the science behind ear corn drying.
We will continue by looking at the drying parameters of air moisture content, temperature and flow to better understand how each contributes to the drying of ear corn.
Drying Parameters
There are (3) fundamental air parameters affecting the drying process.
They are as follows:
Air Moisture Content
For many drying processes, the Air Moisture Content is dictated by the weather conditions and typically not controllable by the dryer operator.
Understanding how changing the weather conditions impacts the drying process os essential for optimum dryer management.
Air Temperature
Air Dry Bulb Temperature has the most impact on the drying process. It is also the most easily controlled by the dryer operator.
Air Temperature and moisture content are interrelated and it is necessary that they be managed in "concert".
Bin Air Flow
"Too much of a good thing" can be counterproductive and, in the case of drying, a waste of resources. Air Flow is generally required in a drying process and is a "good thing" when managed correctly. Knowing when to apply more (or less) air flow can give you the "edge" on drying efficiently.
Airflow is the "vehicle" which transports the water vapor away from the point of evaporation so the rate of evaporation can be maintained. Because there can be many factors that "limit" the rate of evaporation, airflow can be increased to the point where increased benefit is no longer realized and energy is wasted.
Taking an in depth look at the effect of these (3) fundamental drying parameters, we gain insight as to the impact they impose on the drying process.
This in turn gives the dryer operator the understanding they need to make better and more informed management decisions.
The Air Moisture Content Affect
Air Moisture Content or (MC) is the quantity of moisture vapor in a give airspace. MC is represented as the Humidity Ratio on a Psychrometric Chart and commonly quantified as Lb. of Water Vapor Per Lb. of Air (Lbv/Lba).
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MC can be expressed or "quantified" in several ways. For instance, air moisture content is commonly represented in terms of Wet Bulb Temperature (WBT) or Relative Humidity (RH). There are pros &cons of how you "quantify MC and relate it to drying rates. (We will save this topic for another future discussion).
In the chart in Figure 1 above, the vertical axis labeled Humidity Ratio (HR) on the right side of the chart defines the maximum Moisture Content (MC) of the air volume at different air temperatures. Following the horizontal line of the chart from a specific HR values takes you to the Saturation Curve which identifies the air Dry Bulb Temperature that has the energy and volume to accommodate the quantity of MC.
It can be easily seen from the chart in Figure 1 that increased air volume temperature provides for increased MC capacity at a geometric rate.
This is discussed in the next section.
The Air Temperature Affect
The air volume's ability tot hold moisture is determined by its' temperature and this ability increases at a more rapid rate at higher air temperatures. This is illustrated below.
The chart in Figure 2 above illustrates how increasing air temperature increases the ability of the air volume to accommodate increasingly higher quantities of moisture at a. geometric rate.
The Humidity Ratio or (HR) is a measurement of how much moisture a given volume of air is holding or can hold. This is noted on the right side of the above chart as Pounds of Water per Pound of Air (LBw/LBa).
When the air volume is "saturated" with moisture (or at 100% RH), it is holding its maximum amount of moisture in the form of water vapor. The amount of moisture can be determined using the chart above by observing the HR value (right axis) associated with the temperature value on the Saturation Curve to the (100% RH curve).
At 60° F the Humidity Ratio (HR) is .011 LBw/LBa
The Bin Air Flow Affect
Using ear corn as an example of an object being dried by means of Evaporative Cooling, we can begin to understand the role that air flow plays in most convective drying processes. This is illustrated in the three graphics below.
Low Air MC Promotes Increased Dry Rate
The MC of static air surrounding the ear, and the internal MC of the ear corn's kernels and cob establishes a Vapor Pressure "differential" which is called Vapor Pressure Deficit (VPD). At a given temperature, air MC that is lower creates a greater VPD and "pressure" on the MC inside the seed to move the exterior of the kernel and cob for evaporation.
High Air MC Surrounding the Seed Reduces the Dry Rate
As the MC inside the seed migrates through the seed into the surrounding air and is evaporated, it increases the MC surrounding the ear corn. As the air MC now increases the VPD and "pressure" on the MC inside the kernels and cob decreases, moisture migration decreases and the rate of drying slows down.
Air Flow Maintains Dry Rate
The function of air flow is to "renew" the VPD by moving cooler, moisture laden air surrounding the seed ear away and replacing it with hotter, drier air. this continual replacement of the cooler, moisture laden air volume with hotter, drier air serves to maintain the VPD and Dry Rate.
Summary
The management of the drying process dies not have to be "Que sera, sera". ('whatever will be will be'). As the saying goes, "Knowledge is Power". By gaining a better understanding of the science behind drying, the dryer operator is empowered to more optimally manage the drying process.
It can be clearly seen that a more in-depth knowledge base of the basic drying parameters and how they impact seed drying takes some of the guesswork out of dryer management.
P.S. If you are someone who has been looking for this information and want to know more right away, go ahead and message me and I would be more than happy to connect with you.
If there is something you would like me to discuss here, please feel free to reach out and let me know.
Manager-Processing at Rasi seeds | Ex-CORTEVA
3 个月Could you please tell what would be the best air flow rate(m/s) for a double pass drier as well as what should be the optimal flow for single pass drier
Manager-Processing at Rasi seeds | Ex-CORTEVA
3 个月Very informative..,..