Chinese Vacuum Flasks and borosilicate glass – the truth.

We believe it is important to understand the differences between various vacuum tube systems on the market. There are distinct differences in the glass used, durability, and replacement cost of tubes. We are also hearing misleading comments from companies about Chinese panels which are undeserved – in particular, the claim that borosilicate glass will lose its vacuum because of helium penetration.

A Clever System

Borosilicate is commonly known as Pyrex, and can withstand extreme temperatures. In the Chinese vacuum flask system there is a “tube within a tube”, similar to double glazing, but with a vacuum between the two layers. This is a clever system, as it is one piece of glass with no weld or seal holding the vacuum in. The inner tube reaches temperatures of up to 300 deg C in the event of stagnation, which is why borosilicate glass is used.

Other vacuum tubes use a single layer of glass with a small flatplate inside. The glass in this case doesn’t get hot, so other glass such as soda lime glass may be used.

Borosilicate glass v. Soda lime glass

The claim that borosilicate panels would suffer from helium absorption is contradicted by comprehensive tests carried out by G.L. Harding of the University of Sydney. They actually tested vacuum flasks at various temperatures, and found that even in a flask kept constantly at 300 deg C for six months, helium absorption would cause the system to suffer a 2% loss in efficiency (click here to see the paper we are referencing – unfortunately we can’t publish it here ourselves..)

Those proclaiming the benefits of soda lime glass use a single extract (see here) from a book by DeWinter. This quotes an earlier “theoretical formula” which suggested that helium absorption in a panel might cause substantial heat loss after four years. The very next sentence of that book says “The envelope of evacuated collectors …. generally operates at much lower temperatures and would be less vulnerable to helium penetration”.

The many benefits of Chinese tubes

Most consumer products (including the computer I am typing this on) are excellent quality products made in China to exacting standards. We have always made it clear that the vacuum flask systems we sell are made in China. We visited a number of factories, and chose to work with one that met our requirements, both in terms of working conditions for staff, and quality control in production.
We use the flask system because it has many benefits. In particular;

• The vacuum and seal is all contained in one single glass unit rather than having a seal which relies on a connection between different materials.

• Flasks are a lot cheaper and easier to replace than tubes with an integrated heat pipe in the event of breakage or at the end of their life.

• Flasks are made in a number of standard sizes, so replacement units are available from numerous suppliers, rather than being restricted to one supplier for spare parts.

• Systems with a seal that relied on joining metal to glass have had failures over the years.

Don’t be fooled by quoted efficiency figures either – the flask type of tube also has a different pattern of efficiency because the curved shape of the glass means it works differently at different times of the day than a tube with a flatplate.

This information applies equally to many of our vacuum flask competitors. None of us like competition, and it is tempting for European companies to take a swipe at low-cost economies. However, China has had an indigenous solar water heating industry for decades, and produces very simple, efficient, reliable and cost effective water heaters. There are some factories there that don’t come up to scratch, but we have built a very strong relationship with one factory who understand our quality demands and have consistently met the standards which we believe the Irish market demands.

Yours sincerely,

Quentin Gargan

Ecologics Solar Solutions Ltd.,

Kylemore Ind. Est.,

Dublin 10.