On-Vehicle Exhaust Gas Cooling in a Diesel

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Courtesy of DCL International Inc.

ABSTRACT

A diesel emissions control system built and tested on an industrial forklift truck is described. The system utilizes a pleated media type diesel soot filter. The temperature limit of the filter material is well below the temperature of the exhaust gas. Therefore, cooling of the exhaust gas is necessary. A finned-tube heat exchanger is included in the system to cool the exhaust gas to the required level. The performance of the heat exchanger was investigated. The focus was on the fouling of heat transfer surface by soot and the corresponding decrease of the heat transfer rate. The fouling factor was found. Also, a design practice for finned-tube heat exchangers is presented and its applicability to small size mobile equipment is discussed.

 

1. INTRODUCTION

There are several reasons for cooling of diesel exhaust. In some off-highway applications, for example underground mining permissible equipment, the maximum exhaust gas temperature is regulated by law.  In systems with big engines, primarily stationary gensets, the exhaust gas heat recovery may be justified economically. Finally, there exist diesel emission control technologies which utilize heat sensitive materials and are limited to certain exhaust gas temperature ranges only.

 

This paper describes a diesel emissions control system built and tested on an industrial forklift truck. The system utilizes a replaceable pleated-media soot filter with a temperature limit of 200 “C. The temperature of the raw exhaust from the truck’s engine sometimes exceeds 400 “C. The application of the soot filter required a substantial cooling of the exhaust gas.  A finned-tube heat exchanger was installed in the vehicle’s exhaust system and applied as an exhaust gas cooler. The cooling medium was air from the engine’s radiator. Testing of the heat exchanger was performed in the clean system and after 90, 180,260 and 390 hours after the installation.

 

An important issue was the fouling of the heat transfer surface by soot present in diesel exhaust. Two common behaviours are noted in the development of a fouling film over a period of time. One is the so-called asymptotic fouling in which the resistance builds up very rapidly at the start but becomes asymptotic to a steady-state value if conditions remain unchanged. If this kind of fouling occurs, the heat exchanger requires practically no maintenance (cleaning). It is only necessary to overdesign the heat exchanger to such a degree that the heat transfer area will still be sufficient for a given heat duty after the fouling film develops and the heat transfer drops.

 

The other transient behaviour is that of a more or less linear increase in the fouling resistance during the entire operating period. This is encountered mostly when the heat flux is kept constant by increasing the temperature difference. Heat exchangers with the linear increase of fouling resistance require periodic cleanouts and removal of fouling.

 

2. EMISSIONS CONTROL SYSTEM

The emissions control system was installed on a Hyster forklift truck equipped with a Perkins diesel engine.

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