A Quick Overview

The drying of seed corn in ear form (kernels still attached to the cob) at first appears to

be a “continuous” process of moisture evaporation from the kernel and cob. However,

from a seed quality and dryer throughput perspective there is more to it “than meets the

eye”.

Through decades of seed corn drying, considerable understanding gained into different

seed quality risk factors and drying techniques used to mitigation. The Double Pass

Reversing (DPR) dryer design inherently works to preserve seed quality while optimizing

dryer throughput.

The following discussion seeks to give insight about the drying of the seed kernel as the

process transitions from the Wet Stage to the Dry Stage of moisture content in the

production seed corn dryer.

A little background information might be helpful in understanding the “workings” of the

Double-Pass Reversing (DPR) Dryer as it relates to seed quality and dryer throughput.

The DPR Dryer dates back to at least 1938 when a patent was filed by Broman

Campbell of Campbell industries as a new Method For Drying. See Figure 1 below.

The first “objectives” listed in the patent for the new “method of drying” was “simplicity

and economy”. It needs to be noted that the patent illustrates a 4 bin dryer.

The “simplicity” of the new “method of drying” was true of a 4 bin DPR Dryer, however,

as the number of bins in a single dryer increased; the “simplicity” decreased. By today’s

standards, the DPR is often thought of as being complicated to operate due to the need

to maintain the “pressure balance” of the upper and lower air tunnels through bin airflow

manipulation.

Economy is still true in that there are less energy costs with a Double Pass Reversing

(DPR) Dryer as opposed to a Single Pass Reversing (SPR) Dryer. The “double pass”

design uses the heated air twice while the “single pass” design only uses the heated air

once.

Discoveries

The experience gained through the use of the DPR Dryer design over decades of use

has led to the discovery of additional seed quality benefits such as the “tempering” of the

air’s Drying Pressure that occurs on the Down-Air Pass. The evaporative-cooling of the

air exchanges moisture for heat. This “exchange” increases the water content of the air

while reducing it’s temperature thus reducing the vapor pressure that the air exerts on

the seed. You can say that the air has been “tempered” on the Down Air Pass. This

“tempered” air having increased moisture is now used on the Up Air Pass through the

wettest seed corn and is “gentler” to the seed which serves to better preserve seed

quality.

Another discovery that has been “gleaned” through decades of seed drying experience

and related seed drying research is the change in the nature of germ damage risk as the

seed experiences the drying process.

The risks of germ damage at the beginning of the drying cycle or during the UP Air Pass

are different than those toward the end or the Down Air Pass. This phenomenon is a

gradual transition of seed damage risk from damage cause by excessive dry rates to

damage caused by excessive heat. Both seed damage risk phenomena are related

directly to heat so they are often considered and managed as a single risk factor. The

“risk level” change of each phenomena is inverse to that of the other. Through the

drying process the risk due to Dry Rate Sensitivity decreases and the risk of Heat

Sensitivity increases.

These two heat related risk factors need to be assessed and managed independently in

the drying process to minimize seed damage while achieving optimum dryer throughput.

This might be thought of as analogous to the effect of “dehydration” and “heat stroke” to

a human. Both are caused by excessive heat, but each effect causes damage to the

body differently. Consequently they are treated differently. It is the same ins seed corn

drying when it comes to damage caused by “excessive dry rates” and damage caused

by “excessive heat”. These need to be managed differently as well.

With the operation of the traditional DPR Dryer, the process is often managed by

keeping the Lower Tunnel Temperature constant and below what is considered a Quality

Threshold Temperature. This temperature limit is chosen to mitigate the risk due to Dry

Rate Sensitivity. Since the Lower Tunnel Temperature is a function of the Upper Tunnel

Temperature, typically the Upper Tunnel Temperature does not approach the Heat

Sensitivity limit of the seed during the Down Air Pass.

With Single Pass Reversing (SPR) and Hybrid DPR (HDPR) dryers the Down Air Pass

air temperature is managed independent of the Lower Tunnel Temperature. The seed

Heat Sensitivity risk should be managed independently also. Advanced drying

strategies, such as Profile Drying, which can be implemented with SPR and HDPR

dryers, require accurate assessment of the seed variety Heat Sensitivity risk.

Dry Rate Dynamics

For the majority of production seed dryer management strategies, it is the “average” dry-

rate of the seed that is most often considered for seed drying management. In actuality,

the changing dry rate of the seed within a given bin typically resembles the trend

illustrated in Figure 1 below.

The 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 reversal, the

increased air temperature causes an increase in dry rate. However, as the Down Air

Pass progresses and the seed dries, the dry rate continues to slow down.

Due to the difficulty in “measuring” the Dry Rate at any given time in the drying process,

the Average Dry Rate is most often referenced when associating a Dry Rate with a

specific seed variety.

The Average Dry Rate of a bin is typically determined by the total percentage points of

moisture removed from the bin of seed over the period of time air was passed through

the seed pile. Dry Rate is typically expressed in “Hours Per Point” determined by the

following calculation:

(Up Air Hours + Down Air Hours) / % Points of Moisture Removed

While the average Dry Rate may be 3.0 hours /point, 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. This is illustrated in Figure 2 below.

Note that the Dry Rate is greatest at the beginning of the drying cycle as indicated by the

2.0 Hrs/Pt. label. Consequently, it is the seed at the bottom of the bin that is most

susceptible to seed damage due to excessive dry rates.

Dry Rate Sensitivity

As the air enters the seed pile and the evaporative-cooling process occurs, the Dry Rate

decreases as the air passes through the seed pile thus reducing further seed damage

risk.

Typically it is during the first hours (i.e. 5-20 hrs.) of the drying cycle that Dry Rate

Sensitivity may require focused management. Usually by the time a bin reaches the Bin

Reversal Event, Dry Rate Sensitivity is no longer an issue. The first few hours of drying

where Dry Rate Sensitivity is a factor is called the Preconditioning Phase.

During the Preconditioning Phase, the air if often at (or near) saturation as it passes

through the seed pile. This phenomenon was identified earlier in this discussion as the

Drying Front. Seed beyond the Drying Front level in the seed pile is consequently being

subjected to warm, highly saturated air. Warm saturated air will promote both

germination and molding. These conditions need to be avoided, so minimizing the

Preconditioning Phase is an important job of the dryer operator.

The Preconditioning Phase can be shortened by increasing the heat and/or airflow. Both

these solutions will increase the Dry Rate; however, it is at this point in the drying

process where Dry Rate Sensitivity risk is at its maximum. Another solution is to reduce

the bin depth which does not increase the Dry Rate Sensitivity risk.

We can see that it is during the beginning of the seed drying cycle that seed damage risk

requires the most scrutiny and management.

Heat Sensitivity

After the entire seed pile passes through the Preconditioning Phase, the seed may be

subjected to incremental temperature increases. In the traditional DPR there is only one

temperature increase that takes place at the bin’s air direction reversal.

With SPR and HDPR dryers, incremental temperature increases can be enacted more

often as prescribed by the drying management strategy.

JHC’s Profile Drying strategy optimizes the drying time of a bin by incrementally

increasing the air temperature based on a predetermined drying temperature profile for

the specific hybrid or hybrid class.

The Heat Sensitivity characteristics of a given seed variety must be predetermined prior

to implementation of a Profile Drying strategy. This is best done via a lab based process

where a specific seed variety’s drying characteristics are objectively defined via a

controlled drying analytical procedure. This analytical procedure is discussed in

separate documents.

Change Of Focus

As the seed moisture content is removed, first on the Up Air Pass and then on the Down

Air Pass, the risk of seed damage transitions from Dry Rate Sensitivity to Heat

Sensitivity. This is illustrated in Figure 5 below.

As we discussed previously, in a standard DPR Dryer the seed is subjected to a single

heat increment when the bin is switched from Up Air to Down Air. The heat increment is

the difference between the Lower Tunnel Temperature and the Upper Tunnel

Temperature.

The bin Reversal Event is performed at a point in the drying process where the seed has

passed through the Preconditioning Phase (if there is any) and is beyond the risk of

damage due to excessive dry rates.

Up Air Pass and Dry Rate Sensitivity

By examining the psychrometric chart in Figure 6 below, we can better understand the

dilemma that the dryer operator is faced with in mitigating both Dry Rate Sensitivity on

the Up Air Pass and Heat Sensitivity on the Down Air Pass.

The psychrometric chart illustrates that the air’s ability to hold moisture increases

geometrically with temperature. We can see that as the Dry Bulb Temperature of the air

approaches 90F and beyond the Humidity Ratio increases at a more rapid rate.

The higher the seed moisture is when the drying process begins on Up Air the greater

the need for increased moisture holding capacity (Humidity Ratio) of the air. Wetter

seed releases moisture into the air more readily as there are higher concentrations of

moisture near the exterior of the seed. Depending on the amount of water in the

ambient air, the heated air in the dryer often becomes saturated with moisture as it

passes through the seed pile and consequently impedes the dryer process and subjects

the seed toward the top of the seed pile to quality risks.

The saturated air creates a drying phenomenon that is commonly referred to as the

“Drying Front”. The “Drying Front” is the “level” within the seed pile beyond where no

drying occurs due to the air being saturated.

To alleviate this condition, the dryer operator needs to increase the moisture holding

capacity (Humidity ratio) of the air without significantly increasing the seed pile Dry Rate

and thus risking damage due to Dry Rate Sensitivity.

The seed pile Dry Rate is directly related to the Vapor Pressure (far right axis). The

Effective Vapor Pressure is the difference between the “full” moisture holding capacity of

the air and the current moisture content of the air as follows:

Humidity Ratio @ Saturation - Humidity Ratio @ Ambient

Effective Vapor Pressure

The psychrometric chart in Figure 7 below illustrates the effect that the Wet Bulb

Temperature of the air has on the seed pile Dry Rate within a standard DPR dryer.

The green graph line indicates that a Wet Bulb temperature of 75F at a Dry Bulb

Temperature of 90F results in the following:

Net Humidity Ratio = .032 - .015 = .017 (Lbs. Water / Lbs. Air)

Net Vapor Pressure = 1.45 - 0.7 = .075 (Inches Of Hg)

Effective Vapor Pressure as it related to collective Drying Pressure and Dry Rate is

discussed further in an associated document.

Down Air Pass and Heat Sensitivity

After the Bin Reversal Event, the air flow is now passing “down” through the seed pile at

an increased bin inlet temperature. In a standard DPR Dryer the increased temperature

typically remains static.

The risk to seed quality is transitioning to Heat Sensitivity. In a standard DPR Dryer this

risk is typically avoided altogether due to the need to achieve the desired Lower Tunnel

Temperature. Maintaining acceptable Lower Tunnel Temperatures usually requires the

Upper Tunnel Temperature to be comfortably below Quality Temperature Limits.

In the Single Pass Reversing (SPR) and the Hybrid DPR (HDPR) dryer, the Down Air

Pass bin inlet temperature can be increased independently. This being the case, when

drying strategies such as Profile Drying are implemented, the Down Air Pass bin inlet

temperature must be managed to maintain a temperature that is safely below the Heat

Sensitivity Quality Temperature Limit of the specific hybrid.

To Sum It Up

With advancements in ear corn dryer technology and increased understanding of the

“journey” the seed experiences during the drying process, ear corn drying becomes

more manageable and methodical.

What once appeared to be a continuous process of water being evaporated from the

kernel and cob of ear seed corn, is now better understood as a drying process with

multiple phases such as Preconditioning, Dry Rate Desensitizing and the Heat

Tolerance phases.

A clearer understanding of these drying process phases combined with recent

advancements in ear corn dryer technology and dryer management tools such as JHC’s

SDMS - Seed Dryer Management System can all work in concert for optimizing both

the quality and throughput of ear corn seed drying.