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Polymer Under Pressure

01 April 2003

A high-performance polyamide meets the rigours of a domestic central
heating pressure vessel application in this successful plastics-for-steel

The Irish central heating equipment specialist, Ecoplus has been
marketing its pressure reduction tank (PRT) system in the Irish republic
for a couple of years now, and is about to embark on wider distribution
in Europe. When connected to new or existing central heating systems, the
PRT clears oxygen and other gases forming air pockets from the water in
the system. Air pockets can cause a number of problems, including uneven
heat distribution, poor flow rate, noise and corrosion. The Ecoplus unit
is claimed to remove the problem within hours, resulting in an overall
30% improvement in water flow rate and corresponding reduction in energy

Until recently, PRTs were made from steel and weighed almost 30kg apiece.
Ecoplus technical director, Carl Steinborn, however, was on the lookout
for alternative materials of construction and had heard that DuPont's
Zytel HTN polyamide was being used in the manufacture of components for
domestic heating systems. Thus assured of the material's high temperature
and pressure resisting properties, Mr Steinborn got together with
Winchester based Dew Analysis to look at the possibility of using Zytel
HTN for the PRT.

We analysed the design of the PRT, using data from DuPont, to a safety
factor of three. Rapra Technology also ran a series of stress analysis
and fatigue analysis tests, where the material was tested under typical
boiler cycle conditions, and is currently being tested to its ultimate

The Ecoplus PRT is moulded by Ashford, Kent based Ashford Mouldings.
Technical director, Tom Clark, confessed he had not worked with Zytel
before this project, but had subsequently become very impressed with
its high stiffness, good dimensional stability, toughness and overall
structural strength.

During stress analyses, we found Zytel HTN to be about three times as
strong as standard nylon 6/6, yet since it is nylon, it remained flexible
when subjected to sharp changes in hot and cold water, Mr Clark recalls.
As the material can be moulded using relatively low tool temperatures
(80-120°C), Ashford was able to use standard equipment. This would not
have been the case with competing high temperature crystalline
thermoplastics, which usually require very hot, oil-heated tooling.

The Zytel HTN PRT weighs just 4.9kg and is colour-coded by use of an
appropriate masterbatch. The unit has a seven-year warranty, is CE marked
and bears the EnergyStar, a North American eco-friendly certification



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