Embed Size (px)
DESCRIPTION
Grain drying is a process that is influenced by a number of different factors, some of which are more significant than others.
Citation preview
NEXT PAGE
Grain & Feed Milling Technology is published six times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2010 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1466-3872
Digital Re-print - September | October 2010 The influence of temperature, humidity and airflow on the grain drying process
www.gfmt.co.uk
NEXT PAGEPREVIOUS PAGE
Grain drying is a process that is influenced by a number of different factors, some of which
are more significant than others.
These include: 1) the operating plenum tempera-
ture within the grain dryer2) the volumetric airflow rate deliv-
ered by the dryer's fan and 3) the relative humid-
ity of the ambient air. Additionally, there are other less signifi-
cant factors such as the barometric pressure influenced by weather conditions, the eleva-tion above sea level of the dryer site, and the percentage of fines present in the batch of grain to be dried.
This article focuses on the three primary factors of plenum temperature, airflow rate and relative humidity and presents some quantitative data depicting their significance.
Plenum temperature effect on drying rate
The most influential factor that affects the rate of grain drying is the operating plenum temperature.
The operating plenum temperature is significant because it directly impacts the
drying constant and the equilibrium moisture content, two properties that heavily influ-ence the overall drying rate of a given grain. The drying constant primarily governs how quickly the moisture within the grain will be released to the surrounding air.
As the temperature of the grain increase during the drying process, the drying con-stant increases as well, which put simply means it will take less time to remove a point of moisture.
Similarly, plenum temperature influences the equilibrium moisture content of the grain.
By definition, a grain's equilibrium mois-ture content is the moisture that a sample of grain will eventually reach for a given grain temperature and the surrounding air's rela-tive humidity. Because the conditions of the grain in the dryer are continuously changing during the drying process, the equilibrium moisture content of the grain will continu-ously change within a grain dryer as well.
Furthermore, the rate at which the grain reaches the target moisture content at the exit of the dryer changes. This is represented by the slope of the grain drying curve which often times is steeper at the start of the dry-ing process and less steep towards the end. The grain becomes ‘harder’ to dry because
the actual grain's moisture content is closer to the equilibrium moisture.
This reaffirms the concept that the final point of moisture removed from a sample of grain is more difficult than the first point of moisture removed. The equilibrium moisture content of grain is dependent on the tem-perature of the grain.
By increasing the plenum air temperature and transferring that heat to the grain, the equilibrium moisture content of the grain decreases. This means that there is a change in the driving force to dry the grain to a target moisture content.
The graph below shows how the rate of drying changes based on four different plenum temperature scenarios.
This case clearly shows that the operating plenum temperature definitely influences the rate at which the grain releases moisture in the dryer. The incoming moisture to the dryer was established at 22 percent (wet basis) for each of the four scenarios with the ultimate goal of discharging the grain from the dryer at a new moisture content of approximately 18 precent (wet basis).
Each of the four operating conditions require more drying time because of the lower plenum temperature and subsequently lower grain temperature.
The influence of temperature, humidity and airflow on the
grain drying process by Joseph Shulfer, Mathews Company, Crystal Lake IL USA
Grain&feed millinG technoloGy20 | September - october 2010
FeatureDrying
Fan airflow effect on drying capacity
As previously stated, the most influ-ential factor that affects the rate of grain drying is the operating plenum tem-perature, however, another significant factor is the rate at which the heat in the plenum air is transferred to the grain to be dried.
As the heated air passes over the grain in the column of the dryer, convective heat transfer is taking place between the hot air and the relatively cold grain. The rate of heat transfer is governed primarily by the volumetric airflow rate and the temperature difference between the grain and air.
Because the rate of grain drying depends on the drying constant and the equilibrium moisture content of the grain (both of which are heavily influenced by the plenum air temperature), the faster the heat can be transferred to the grain, the faster the grain will give up its moisture to the air and ulti-mately reach the target moisture content.
Therefore, the grain drying rate is influ-enced by the airflow rate delivered by
the dryer's fan because it directly impacts the rate at which the plenum heat is transferred to the grain.
Typically, a grain dryer's fan airflow perform-ance is based on the volume of airflow that crosses the grain column in a given period of time. This is measured in airflow per volume of grain or CFM/bushel. Calculations based on grain drying principles of airflow, heat transfer, mass transfer, and ther-
Grain&feed millinG technoloGy September - october 2010 | 21
Feature Drying
Now distributingMarot rotarycleaners and
spares
Chief dryers Chief silos
You can trust in Chief.
Grain.It’s your business.
Beckingham Business ParkTolleshunt Major, MaldonEssex CM9 8LZ, UK
Tel +44 (0)1621 868944Fax +44 (0)1621 868955E-mail [email protected]
Which is why we neverunderestimate the importanceof how it is handled.If you need a partner with theexpertise, technology andmanufacturing methods toensure that your storage plantis second-to-none in terms ofquality and processes, thenlook no further.
Quality grain handling
idea that two of the three can be adjusted for optimal performance, plenum tempera-ture and airflow rate. Humidity is the only factor that is outside of a user's ability to control, however adjustment of temperature and fan airflow rate may offset the humidity effect on drying.
When selecting the optimal point for grain dryer operations, consideration needs to be given to the plenum operating tem-perature.
Although the data would suggest that high-er plenum temperatures will result in higher drying rates, grain quality, the potential of a fire and operating costs need to be considered.
Nearly all modern grain dryers will have some means to adjust plenum temperature either manually or automatically and even more precise controls such as digital tem-perature control are available as in the case of all Mathews Company dryers.
Fan airflow is usually established by the grain dryer manufacturer and remains fixed unless the control system utilizes a variable frequency drive (VFD) to increase or decrease the fan's speed, which will increase or decrease the fan's airflow rate. Also, in the case of a vacuum cooled dryer in which the free air flow on the suction cooled side is adjustable, varying dry-ing rates may be achieved at the expense of elevated grain temperatures.
The information presented here is by no means a recommendation on how best to operate a grain dryer, yet it is important to understand the influences that different operational and environmental factors have on the ability of a grain dryer to do its job. Grain drying and the science behind it has been around for many years.
New advances in control systems which leverage the information contained within this article, are helping grain dryer manufac-turers like Mathews Company design, build, and deliver the most state-of-the-art grain dryer for the industry.
to understand that although the relative humidity of the air being used to dry grain will adversely affect the drying rate of grain, what matters more is the humidity of the air when it passes over the grain.
Fundamentally, a dryer works because the incoming air is heat-ed to an elevated temperature and subsequently increases the temperature of the grain which makes the moisture removal process of the grain easier.
Unfortunately, this increase in air temperature does not help with the air-water transfer phe-nomenon that takes place during the drying process.
Based on psychrometric princi-ples, as humid air is heated up (in the case of a grain dryer) the relative humidity will decrease because the difference between the wet bulb and dry bulb temperature changes, however, the amount of water vapor relative to the amount of dry air will not change.
Since the driving force of water being transferred from the grain to the surround-ing air depends on the humidity ratio (ratio of water vapor to dry air) which remains unchanged as the air is heated up, the temperature increase of the air/grain will not alleviate the effects of humidity on grain dry-ing. Furthermore, as the air travels through the column of grain and water is transferred from the grain to the air, the relative humid-ity and humidity ratio will increase making it more difficult to dry grain on the outer edges of the dryer because the temperature of the air is lower and the moisture of the air is higher.
Other considerationsThe three primary factors influencing the
rate at which grain dries in a continuous flow grain dryer have been presented with the
modynamics predict the influence of fan airflow on grain drying rate for a typical all-heat dryer operating with incoming mois-ture content of 25 percent as shown in the accompanying graph.
Ambient air humidity effect on drying rate
Of the top three factors that influence the rate at which grain will dry in a continuous flow grain dryer, the ambient air's relative humidity is the least significant.
Furthermore, unlike the operating plenum temperature and the fan airflow rate, a dryer operator really has no control over the humidity.
Certainly, the relative humidity of the ambient air is heavily influenced by geo-graphic location, but it can also change over the course of the year depending on what season the dryer is operating.
Operationally, there is not much that can be done to avoid the effects of humidity on the performance of a dryer because the higher humidity conditions equate to a higher amount of water vapor mixed with dry air. It is important
About the AuthorJoseph Shulfer joined Mathews Company in 2009 and is responsible for all engineering and field service activi-ties. Under the direction of Joe, his team of engineers and field service technicians design, test, and service the latest technology of grain drying equipment that Mathews Company has to offer.
Prior to joining the Mathews Company, Joe held the role of engineering product manager for Eisenmann Corporation where he successfully developed and managed a product line which encompassed environmental and renewable energy solutions for the first generation corn-based ethanol plants throughout the Midwestern United States. Joe is a licensed professional engineer, holds a BS in Mechanical Engineering, and an MBA from the Webster School of Business and Technology. Mr Shulfer can be contacted by email: [email protected]
Mathews Company (www.mathewscompany.com) is a leading, family-owned manufacturer with nearly 60 years experience building grain drying products and is based in Crystal Lake, IL. The mission of Mathews Company is to design and build innovative, high quality equipment by way of engineering excellence and world class manufacturing. Our goal is building relationships that last with our dealers, customers and employees.
Grain&feed millinG technoloGy22 | September - october 2010
FeatureDrying
Grain&feed millinG technoloGy September - october 2010 | 23
With our Heat Recovery Dryer you get the lowest energy consumption on the market.
Read more about our product range and news on www.tornum.com
Up to 30%Energy Savings
Heat Recovery Dryer
TORNUM AB Box 100, 535 22 Kvänum, Sweden Phone +46 (0)512–291 00 e-post [email protected]
www.tornum.com
The solution behind the solution.
Fatten up your bottom line. Buhler high-performance animal and aqua feed production systems are used by leading companies around the world. These producers know they can rely not just on the technology itself, but also on the support that accompanies it. A service combining local presence with global expertise both lowers feed mill operating costs and increases capacity utilization. So the question is not whether you can afford to choose Buhler – it’s how a solution from Buhler will feed your profits. To find out more, visit www.buhlergroup.com.
Bühler AG, Feed & Biomass, CH-9240 Uzwil, Switzerland, T +41 71 955 11 11, F +41 71 955 28 96
[email protected], www.buhlergroup.com
Grain_Feed_Sept-Oct.indd 1 15.07.2010 16:28:18
PREVIOUS PAGEwww.gfmt.co.uk
LINKS• Seethefullissue• VisittheGFMTwebsite
• ContacttheGFMTTeam
• SubscribetoGFMTA subscription magazine for the global flour & feed milling industries - first published in 1891
September 2010
• Thebenefitsofextrusioncooking
In this issue:
• The law of diminishing
returns: - consequences
for feed enzyme strategy
• An innovative approach to animal diet formulation
• Global grain & feed markets
• Cutoperatingcosts!
- Energy savings in drying technology for agricultural products
• Theinfluenceoftemperature,humidityandairflowonthegraindryingprocess
ThisdigitalRe-printispartoftheSeptember|October2010editionofGrain&FeedMillingTechnologymagazine.Contentfromthemagazineisavailabletoviewfree-of-charge,bothasafullonlinemagazineonourwebsite,andasanarchiveofindividualfeaturesonthedocstocwebsite.Pleaseclickheretoviewourotherpublicationsonwww.docstoc.com.
Topurchaseapapercopyofthemagazine,ortosubscribetothepapereditionpleasecontactourCirculationandSubscriptionsManageronthelinkadove.
INFORMATIONFORADVERTISERS-CLICKHERE
Article reprintsAll Grain & Feed Milling Tecchnology feature articles can be re-printed as a 4 or 8 page booklets (these have been used as point of sale materials, promotional materials for shows and exhibitions etc).
If you are interested in getting this article re-printed please contact the GFMT team for more information on - Tel: +44 1242 267707 - Email: [email protected] or visit www.gfmt.co.uk/reprints
