CSD_PSY 401 - Dry Rate Dynamics

Overview

Any dryer operator who has managed an ear corn seed dryer for very long has observed that the drying process is not a “linear” process.

While the rate at which a bin of seed corn dries is generally referred to as “static” rate such as 3.75 Hours Per Point Of Moisture (3.75Hrs./Pt.), in reality this is the “average” rate of drying over the course of time the bin was “On Air”.

The Seed Dry Rate affects both seed quality and dryer throughput; however, the seed drying process is generally not managed from a “dry rate” perspective. This is changing.

The objectives of this article is to examine the “pros and cons” of “traditional” ear seed corn drying practices and provide an introduction to advanced drying strategies that are based on the methodical control of Seed Dry Rate.

“Que Sera, Sera”

Many are familiar with the title to the 1956 Doris Day song, "Que Sera, Sera", (Whatever Will Be Will Be). This could also be the “title” of the many dryer management strategies which fall into the “traditional” category of drying management. The dryer bin’s airflow is pretty much “fixed” due to fixed speed fans and there is an Up Air Temperature and a Down Air Temperature that is “set” as well. It is considered part of the dryer operator’s job to maintain these temperatures.

We have discussed in our article titled Seed Moisture Migration that the air volume’s Vapor Pressure Deficit (VPD) and seed physiology’s Migration Impedance (ZM) are the fundamental factors influencing seed moisture migration and evaporation rates. In turn, these factors work together to establish the seed’s rate of drying.

Using an ear seed corn drying strategy that does not incorporate active dry rate management “allows” the Seed Dry Rate to vary based on ambient air moisture content and different seed varieties that have distinctly different physiology characteristics.

Consequently, as far as the Seed Dry Rate is concerned, “whatever will be will be”. The “Que Sera, Sera” strategy has worked for decades when the seasonal capacities and operating costs of the drying process were less of an ROI issue.

However, in today’s “Big Ag” business environment where often times the seed producer is either competing with, or competing to grow for, “Big Ag”, superior utilization of seed dryers and reduced operating costs provide a competitive edge in seed corn production profitability.

Seed Dry Rate “Dynamics”

While it is “common knowledge” that the seed in a production dryer bin is not drying at a constant rate, there has not been a “convenient” method to determine the “actual” Seed Dry Rate at any given time in the drying process. This has been a major reason for using the “average” Seed Dry Rate as primary dryer management criteria to assess “in process” seed quality and dryer throughput.

In actuality, the changing dry rate of a given bin of seed corn in a Double Pass Reversing (DPR) or Single Pass Reversing (SPR) dryer typically resembles the trend illustrated in Figure 1 below.

In this illustration, during the Up Air Pass the “actual” dry rate starts out significantly greater than the “average” and then slows to significantly less than the average at the time of bin reversal. After bin airflow reversal and increased air temperature, an increase in dry rate is achieved. Then on the Down Air Pass, as the seed dries, the dry rate again decreases.

The average Seed Dry Rate may be 3.0 hrs./pt., however, the seed may experience dry rates of double that at the beginning of the bin drying cycle and half, or less, at the end of the drying cycle.

Earlier In this article we stated that, “the air volume’s Vapor Pressure Deficit (VPD) and seed physiology’s Migration Impedance (ZM) are the fundamental factors influencing seed moisture migration and evaporation rates”. We also stated that, “these factors work together to establish the seed’s rate of drying”.

Referring to the illustration of Figure 1, if we assume that the weather conditions were somewhat static, then the Vapor Pressure Deficit (VPD) of the air volume is somewhat static as well and is already “factored” into the curve that represents the “actual” dry rate values.

What remains as the primary Seed Dry Rate “influencer” is the seed physiology’s Migration Impedance (ZM). What is occurring is that the seed’s Migration Impedance (ZM) is increasing as the seed dries.

This being the case, we can see that in order to compensate for the change in Migration Impedance (ZM) and maintain a more consistent dry rate, a smaller VPD is needed at the beginning of the drying cycle where “actual” Seed Dry Rate is noted as 2.0 Hrs./Pt and a larger VPD is needed at the end of the drying cycle where “actual” Seed Dry Rate is noted as 4.0 Hrs./Pt..

Seed Dry Rate “Metrics”

A fundamental process management axiom is “if you cannot measure it, you cannot manage it”.

In the case of Seed Dry Rate there is no Seed Dry Rate “sensor” currently on the market, so there is no method to directly measure it.

However, as stated earlier, if the seed physiology’s Migration Impedance (ZM) is known, we can manage Seed Dry Rate by controlling Vapor Pressure Deficit (VPD). We can measure VPD indirectly using a Wet Bulb Temperature (WBT) sensor and correlating Wet Bulb Depression (WBD) to Vapor Pressure Deficit (VPD).

Wet Bulb Depression (WBD) is simply the difference between Dry Bulb Temperature (DBT) and Wet Bulb Temperature (WBT).

The Wet Bulb Temperature (WBT) is the most cost effective and intuitive method of “gauging” and “managing” Seed Dry Rate for the following reasons:

1. Wet Bulb Temperature (WBT) sensors are readily available, “off the shelf”, sensing components.
2. The ear seed corn drying process is accomplished via evaporative cooling. In this process “energy” is not lost but changes form. It begins as ”sensible heat” (heat that can be felt) and changes to “latent heat” (heat held by the evaporated moisture) as a result of the moisture’s “phase change” into a vapor. As this “energy transformation” occurs, this water-vapor “change” follows the Wet Bulb Line of the Psychrometric Chart.
3. Only a single WBT sensor is needed in a DPR dryer. Once the heated air enters the DPR dryer, the Wet Bulb Temperature does not change. For SPR dryers, individual bin WBT sensors are needed because individual bins may be heated to different DBTs. (JHC offers specialized, cost effective, multiplexed WBT sensors for SPR dryers with our SDMS - Seed Dryer Management System.)

There are several advantages to utilizing the Wet Bulb Temperature (WBT) and JHC’s WBT Dryer Management Strategy to manage ear seed corn drying in DPR and SPR dryers. The Wet Bulb Temperature (WBT) and the advantages of JHC’s WBT Dryer Management Strategy will be discussed in more detail in an upcoming article.

Managing VPD Using WBD

Another significant factor affecting the Seed Dry Rate is weather conditions. As weather conditions change, the water content of the ambient air volume changes accordingly. This affects the ambient Vapor Pressure Deficit (VPD) and the VPD inside the dryer as well.

An air volume’s Vapor Pressure Deficit (VPD) value can be correlated to Wet Bulb Depression (WBD) and then managed using the Wet Bulb Temperature (WBT) value.

This can be accomplished by adjusting the Fan-Burner System (FBS) temperature to the value necessary to achieve the desired Vapor Pressure Deficit (VPD) as indicated on the Psychrometric Chart as dictated by the current Wet Bulb Temperature (WBT) value.

As an example, if the “nominal” drying environment in the lower tunnel of a Double Pass Reversing (DPR) dryer is designated to be a DBT of 90°F (32.2°C) at a WBT of 70°F (21.0°C), then the “target” VPD would be 0.683 In.Hg. as listed in center, highlighted row of Table 1 below.

By monitoring the WBT and adjusting the Fan-Burner System (FBS) temperature according to the Table 1 above, the VPD can be normalized to be near the “target” value of 0.683 In.Hg.

Continuing the example and referring to Table 1, if the wet bulb temperature changes from 70°F (21.0°C) to 75°F (23.9°C), then the Fan-Burner System (FBS) temperature should be adjusted from 90°F (32.2°C) to 92.5°F (33.6°C) to maintain the approximate “target” VPD of 0.663 In.Hg.

These adjustments for changing weather conditions could be made manually or they can be made automatically by a dryer management system such as JHC’s SDMS - Seed Dryer Management System. An introduction to this feature of SDMS is discussed below.

The Drying Profile Concept

The Drying Profile can be thought of as the “recipe” by which SDMS manages the drying parameters in order to maintain the optimum dry rate for the seed in the dryer at all times.

As the seed progresses through the drying process, the dryer management system refers to the Drying Profile designed for the specific seed variety to determine when and how to make adjustments to the drying parameters.

There are (3) dominating factors that affect the Seed Dry Rate of a particular corn variety.

1. Air Properties
2. Air Management
3. Seed Characteristics

Air Properties refers to the physical and thermodynamic properties of the gas-vapormixture that makes up the air. This includes air temperature and moisture content. Psychometrics is the field of science which defines this.

Air Management is the control of the air volume and/or speed of the air that is directed through the seed pile in a particular bin of seed corn.

Seed Characteristics denotes the characterization of the seed’s capacity for internal moisture to migrate to the seed exterior without causing damage to the seeds germination or vigor. This factor is intrinsic to the seed physiology and cannot be altered. The seed physiology “impedes” moisture migration so we refer to this characterization as the seed physiology’s Migration Impedance (ZM).

This concept is illustrated in Figure 2 below

SDMS Profile Drying Feature

The Seed Drying Characterization for a specific seed variety is generally developed based on data from laboratory drying experiments involving the controlled drying of the seed while monitoring the quality.

Once the drying characteristics of the seed have been determined, the drying profile for each unique seed variety can be designed and then entered into SDMS for assignment to the seed varieties that have similar Seed Drying Characterizations.

One of the SDMS Drying Profile Management dialog box is shown in Figure 3. This is where the designed profile data is inputted and edited.

Summary

We can see that the saying that “Knowledge Is Power” is true in the ear seed corn drying process. A superior understanding of the dynamic nature of the dryer’s air-volume Vapor Pressure (VP) and the dryer management strategies to maintain Seed Dry Rates become valuable “assets” to the dryer manager that desires to optimize the ear seed corn drying process.

With advancements in ear seed corn dryer sensing and control technologies, combined with advanced dryer management software systems such as JHC’s SDMS - Seed Dryer Management System, dryer performance, efficiency and throughput can be significantly improved without sacrificing seed quality.

Jacobsen Holz Corporation specializes in innovative Ear Seed Corn Dryer products and services. We draw from over 4 decades of experience in the seed corn production arena to design and provide drying solutions for our customers at home and abroad.

Happy New Year!

I have thoroughly enjoyed all of the kind responses and inquiries.

If you are interested in learning more about some of the concepts I have discussed in this and the past articles, please feel free to reach out to me. I will do my best to answer in a timely manner.

My team and I are working to bring you even more value in this new year.

We are excited about what we have in store.

Wishing everyone a successful start to your year. I look forward to connecting with everyone.

Thank you,

Joe Jacobsen