LPG Discussion Forum

Don't know why I should care, or that it ever matters, but this, and i shall dub it a 'Double T' fitting, is what you find plonked in the coolant hose on factory BiFuel Volvos...



Never seen such a thing before or since. It sits only on the >>flow>> to the heater. Being a 'Double Tee' it doesn't straddle the flow and return lines, as with usual use of a 'Single tee x 2" set-up. This Double Tee will clearly offer less 'flow', but it's tidier. One cut in the hot feed of your coolant-hose feeds your reducer. Not two.

But if getting hot water to your reducer/ vapouriser is one fitting, why is this not standard practice? Or is it that - this from a CNG car - the hot-water feed requirement is much lower?

I just like to know this bollards...

I would suspect it isn't normally used as I can't see there being much, if any, flow through the vaporiser. Think about it, why should the coolant flow out of the tee to the vaporiser when it can take a much easier path through the heater? Come to that, even if there was a lot of restriction through the heater, the return comes back to the same place so even then, why would it flow through the vaporiser? If it was plugged in the centre, it would put the vaporiser in series with the heater (my preferred method) so it would get full flow and would make far more sense. But if that was the case, I'd expect all pipes to be the same size?

I can't see there being much, if any, flow through the vapouriser. Think about it, why should the coolant flow out of the tee to the vaporiser when it can take a much easier path through the heater?

From a different direction, that would be my thinking too. Hence this thread. And we'd be wrong. Because it works, and works well. This from my V70 CNG conversion thread, it also worked well when it was found with a Zavoli LPG vaporiser fed via the same 'Double T'. If it were a 'Double Elbow', thus the reducer was in series with the heater which here it isn't I'd accept that. But it isn't, and still it works.

With the vapouriser/reducer at a 'dead-end' it can't 'flow' as such, reliant purely on convection and the venturi effect, which must be more significant thatn you'd think? Oh well, it's a success, thus reckon I'm on sticky ground to argue with Necam/Volvo's R&D team.

Probably only used on CNG bi fuel models (as opposed to LPG bi fuel models)... Lpg reducers need more heat than Cng reducers, most Lpg reducers tend to have 16mm water connectors, Cng 10mm water, though there are plenty exceptions for both types.

Clearly it will work if there is a 10mm restriction right in the middle.

Yes, that restriction in particular makes no sense whatsoever, why in hell is it there? Only there it is, and it works. What with the restriction, by all rights this seems designed to ensure it WON'T work. And at the very least places a greater restriction on supply to the heater?

Simon you confirm my thoughts that LPG will require more 'flow' over its CNG brethren. Which begs the question,
when I can't see harm in adding extra flow to a CNG reducer, why would Necam/Koltec make two differing fittings, one LPG, one CNG (with more restriction?) - if indeed they did? This makes no sense to me, hence this thread.

Maybe if I could remember my fluid dynamics, that restriction is enough to force some flow, thus equalise flow both via the reducer, and thru' the restriction? Hence this 'Double tee' is actually better than the standard issue - 2 x tee - in flow and return, which forces some heat to be shared with the heater in parrallel. The Double T can be plumbed to get heat first?


I'll leave better than me to fathom.

I think you've answered your own questions there mate, like I said the restriction is there to force some water to take the narrower route through the CNG reducer.

It is just manufacturer convention to fit 10mm coolant connectors on CNG reducers... You've asked why 2 different sizes of pipe/connectors are used for CNG/LPG, I suppose looking at it from another angle we might otherwise ask why manufacturers would fit 16mm pipes/connectors on a CNG reducer when they don't need that much flow. LPG reducers need more flow than CNG reducers because the change of phase from liquid to vapour LPG causes more of a temperature drop than just reducing vapour pressure with no 'change of phase'.

On some LPG installs (with 5/8 16mm connectors) we plumb the reducer in series with the heater matrix but we wouldn't want to plumb a reducer in series with the matrix if the reducer only has 10mm connectors because it would restrict flow through the matrix in series.

The double T you've pictured would see the CNG reducer with it's 10mm connectors get enough flow even if that hole in the middle of the double T were completely closed because that would mean the reducer would be plumbed in series with the (wider) matrix but then the matrix wouldn't get as much flow as it usually does. With the restriction in the double T the reducer is effectively plumbed in series but with a bypass to allow the matrix to get it's full flow (half going through the bypass which is in series with the matrix, half going through the reducer which is in series with the matrix). If there were no restriction the reducer might get little to no flow because the majority of flow would go through the much wider and shorter 16mm hole instead of taking the longer route through 10mm holes.