ICS 65.060.01  
 
Reference number  
DRS  268-4: 2020  
© RSB 2020  
  
 RWANDA 
STANDARD  DRS  
268-4  
 
2nd edition  
2020 -June  
 
Agroprocessing  machine s — 
Specification —  
Part 4:  
Heated air mechanical grain dryer  
   
DRS  268-4: 2020  
©RSB 2020  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2020  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reprodu ce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw   DRS  268-4: 2020  
iii                ©RSB 2020 - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Classification  ................................ ................................ ................................ ................................ .........  2 
4.1 System of operation  ................................ ................................ ................................ ..............................  2 
4.2 Heating system  ................................ ................................ ................................ ................................ ...... 3 
5 Requirements  ................................ ................................ ................................ ................................ .........  3 
5.1 Performance  ................................ ................................ ................................ ................................ ...........  3 
5.2 Safety  ................................ ................................ ................................ ................................ ......................  4 
6 Workmanship and finish  ................................ ................................ ................................ .......................  5 
7 Maintenance and operation  ................................ ................................ ................................ ..................  5 
8 Sampling and testing  ................................ ................................ ................................ ............................  5 
Marking  ................................ ................................ ................................ ................................ ................................  5 
 DRS  268-4: 2020  
©RSB 2020  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and a pplication of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment  as Rwanda Standards.  
DRS  268-4 was prepared by Technical Committee RSB/TC 047, Steel aluminium and related products . 
In the preparation of this standard, reference was made to the following standard  (s): 
1) XYZ: Title  
2) XYZ:  Title 
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second  edition cancels and replaces the first  edition (RS  268-4: 2015 ) which has been technically 
revised.  
DRS  268 consists of the following parts, under the general title Agroprocessing machinery — Specification : 
 Part 1: Rice thresher  
 Part 2: Power operated maize sheller  
Part 3 : Rice mill  
Part 5: Maize mill  
Committee membership  
The following organizations were represented on the Technical Committee on Steel alminum and related 
products (RSB/TC 047) in the preparation of this standard.  
University of Rwanda/college of science and technology  
University of Rwanda/College of agriculture animal science and veterinary medicine  
Kabizu business group Ltd  DRS  268-4: 2020  
v                ©RSB 2020 - All rights reserved  
 Rwanda Polytechnic/IPRC Kigali  
Rwanda Polytechni c/IPRC Ngoma  
Rwanda Polytechnic/IPRC Musanze  
RWANTECH Boilers  
Rwanda Inspectorate and competition authority  
Rwanda Institute for Conservation Agriculture  
ACER Ltd  
Rwanda  Standards Board  (RSB) – Secretariat  
  DRS  268-4: 2020  
1                ©RSB 2020 - All rights reserved  
 Agroprocessing  machine s — Specification — Part 4: Heated air 
mechanical grain dryer  
1 Scope   
This Draft Rwanda Standard specifies the classification, requirements, methods of test and sampling for 
heated air mechanical grain dryer used for commercial purposes. It does not include dryers for seeds  
2 Normative references   
The following documents are referred to in the text in such a way that some or all of their content constitutes 
requirements of this document.  For dated references, only the edition cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies.  
DRS 269 -4, Agroprocessing machines — Test methods — Part 3: Rice mill    
RS 241, Agriculture  machine ry— Methods of sampling   
RS 236, Acoustics — Noise pollution — Tolerance limit  
3 Terms and definitions   
For the purposes of this standard, the follow ing terms and definitions apply  
3.1  
batch type  
mechanical grain dryer where in the grain in fixed volume is held in the drying chamber in batches until the 
grain reaches the desired moisture content  
3.2  
continuous flow dryer  
dryer  in which the material being dried moves through the drying chamber in a substantially continuous stream 
and is discharged without being recirculated  
3.3  
cracked grains  
grains which show signs of fissures or fractures or splinters   
©RSB 2020  - All rights reserved                  2 
 3.4  
dryer, direct -fired   
dryer in which the products of combustion come into direct contact with the product being dried  
3.5  
dryer, indirect -fired  
dryer in which the products of combustion do not come in contact with the products being dried   
3.6  
head rice  
grain or a fraction of grain with its size equal to or greater than eight -tenth (8/10) of size of the whole grain  
3.7  
heated -air mechanical grain dryer  
device used to remove grain moisture by forcing heated air through the grain mass until the desired moisture 
content is at tained  
3.8  
moisture gradient  
difference between the maximum and the minimum grain moisture content randomly sampled after drying.  
4 Classification  
Heated -air mechanical grain dryers shall be classified based on:  
4.1 System of operation  
Heated – air mechanical grain dryers based on system of operation shall be as follows:  
a) batch type; and  
1) flat bed: shallow bed batch type dryer wherein a fixed volume of grain is held stationary in a horizontal 
grain holding bin  
2) recirculating: batch type dryer equipped to circulate and/or mixed fixed volume of grain during the drying 
operation; and   
3) vertical bin: columnar type batch type dryer wherein a fixed volume of grain is held stationary in a 
vertical grain holding bin  DRS  268-4: 2020  
3                ©RSB 2020 - All rights reserved  
  
b) continuous flow  
1) concurrent -flow: parallel flow type continuous flow dryer wherein the product being dried moves  in 
the same direction as drying air  
2) counter -flow: continuous flow dryer wherein the grain being dried move in one direction and the  
drying air moves in the o pposite direction  
3) cross -flow: continuous flow dryer wherein the flow of air is transverse to the direction of flow of the 
grain being dried  
4) Mixing: continuous flow dryer wherein the drying bin is similar to columnar drying bin except that it 
includes lou vers causing mixing to occur as the grain flows through the system  
5) non-mixing: continuous flow dryer wherein the grains in the drying bin flows through the column in a 
straight path.  
NOTE It consists of two parallel screens or columns of louvers. The spa ce between the two columns is the plenum 
chamber where heated air is introduced and forced through the grain  
4.2 Heating system  
Heated – air mechanical grain dryers based on heating system shall be as follows:  
a) method of heat introduction; and  
1) direct  
2) indirect  
b) fuel source  
3) conventional  
4) non-conventional  
5 Requirements  
5.1 Performance  
5.1.1 The performance of heated -air mechanical grain dryers shall be as specified in Table 1.   
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 Table 1 — Performance criteria for mechanical grain dryer  
S/N Criteria  Rice Maize  
Batch/(Continuous Flow)  Batch/(Continuous Flow)  
1.  Final moisture content, % w.b  14.0 14.0 
2.  Moisture gradient, %, max.  2.0  2.0  
3.  Product quality*  
- Cracked grain, % increase, max.  5.0 (2.0)  35 
- Head rice, % decrease, max.  5.0 N/A 
- Hulled/damaged grain, % increase, max.  3.0 N/A 
- Brokens/Split kernels, % increase, max  N/A 7.0 
4.  Spillage, %, max.  0.5 0.5 
5.  Drying efficiency, % min.  75 75 
6.  Heating system efficiency, % min  
Petroleum based fuel direct  fired 90 90 
Petroleum based indirect -fired 75 75 
Biomass fuel direct -fired 65 65 
Biomass fuel  indirect -fired 50 50 
* allowable difference between the laboratory analysis and machine’s performance  
 
5.1.2 The indicated grain holding capacity must be attained.  
5.1.3 The dried grain shall have no additional discoloration, no traces of unburned fuel or ashes on grain 
surface and no fermented or musty smell.  
5.1.4 The dryer shall be provided with thermometer to measure the actual air temperature entering the grain  
mass and a pressure gauge to measure the working static pressure in the plenum.  
5.2 Safety  
5.2.1 It shall have adequate provision for fire control.  
5.2.2 It shall have adequate protection from or for all moving parts. All rotating parts shall be dynamicall y 
balanced.  
5.2.3 It shall be provided with features for access to parts during repair, maintenance and operation.  
5.2.4 The noise emitted by the heated -air mechanical grain dryer shall comply with the requirements given in 
RS 236   
5.2.5 Provision shall be made for dust control. Its dust emission shall be within the acceptable limits set by the 
National Authority in charge of Environmental management. The emission shall be controlled and managed  DRS  268-4: 2020  
5                ©RSB 2020 - All rights reserved  
 6 Workmanship and finish  
6.1 Heated -Air mechanical grain dryer  shall be free from defects that may be detrimental to its use and shall 
be free from sharp edges and surface that may hurt the operator.  
6.2 All metal parts shall be machine bend, pressed and cut to avoid rough surfaces and all rough surfaces 
shall be ma chine finished and smoothed.  
6.3 Uniformity of parts and components for same brand and model shall be maintained  
7 Maintenance and operation  
7.1 Each heated -air mechanical grain dryer shall be provided with a set of relevant mechanic tool kit.  
7.2 An ins truction manual (User’s manual, parts catalogue and service manual) shall be provided.  
7.3 All components that require regular maintenance, servicing and adjustment should be easily accessible  
 
8  Sampling and testing  
8.1 The heated -air mechanical grain dr yer shall be sampled for testing in accordance with RS 241.  
8.2 The sampled heated -air mechanical grain dryer shall be tested in accordance with RS 269 -4 
Marking   
Heated air mechanical grain dryer shall be indelibly and legibly marked with the following information in one of 
three official languages used in Rwanda:   
a) registered trademark of the manufacturer;  
b) type; serial number;  
c) name and address of the manufacturer;  
d) country of manufacture;  
e) load capacity, tons;  
f) rated power/voltage/frequency/phase;  
g) dry weight, kg;   
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 h) dimension; and  DRS  268-4: 2020  
 
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