WoodlandsCamper said:
Call it what you like, a radiator or a towel rail, it still needs an IN and an OUT for the flow of water to transfer the heat. Try turning a domestic radiator off at one end - it don't work, because there isn't a flow !!
hmy:
High Woodlands
I haven't seen this particular heated towel rail in the flesh, so what I'm about to write is based on a well reported dynamic effect of moving fluids in contact with a surface, the "Coanda" effect. This is where a jet of fluid will 'attach' to a surface, and will tend to follow the path of the surface until its velocity reduces or something else disturbs it.
You can easily see this effect if you turn on a tap, and a spoon vertically and make the back of the spoon touch the falling water. The water will start to follow the curve of the spoon. This is the same effect that allows aircraft wings to work.
In the towel rail scenario, the flow of hot water in the CH pipe will find the the TEE coupling and teh inner surface of the coupling will curve into the towel rail riser pipe. The velocity of the water flow will cause the water in close contact with tee will try to follow the curve of the fitting and thus sme hot water with a velocity will find its way into the riser pipe. As the pipe is of a modest diameter (About 22mm) there is enough csa to allow hot water to move up one side if the riser, whilst cooler water can move down the other side. The hot water will have velocity from its initial thrust from the CH circulation, but also as the pipe is vertical, it will acquire a buoyancy over the cooler water in the rail. The design of the rail will encourage a circulatory effect as the hot water tends to rise to the top directly through the tall riser, but as the water loses its heat and buoyancy it will tend to fall by its increased density down the 'P' leg, rejoining the lower part of the riser, where a bi directional laminar flow takes place within the short length of pipe.
This process will enable the rail to function, but the rate at which heat can be transferred is relatively low compared to a full flow radiator. It will only work if the towel rail can lose enough heat to cool the water to raise its density enough to start the convection effect.
There are tricks the manufacture cold use to improve the flow, initially simply inserting a long springy blade inside the lower part of the riser to divide the pipe internally to create hot in and cold out sides pipe. If the same blade also protruded through the TEE coupling and into the CH pipe some way it would help to divert more flow into the rail.
The more recent adaption of the twin TEE piece and the distinct in and out pipes is a variation of the same process. which has often been used in large scale heating systems in buildings. Sometimes there is a small restriction in the tee piece sections which will encourage water to divert through the radiator loop.
A totally different approach where a single pipe closed at both ends can transfer a lot of heat was used in the Carver Cascade Mk1 (Copper mushroom) heater from the 1980's
This used a two phase thermal heat pipe which infact is a deceptively simple but very effective and reliable process. Heat is used to vapourise a fluid, but them by allowing the vaspour to lose its heat it will condense back to its liquid state.
I saw this component being manufactured at Carver's factory. I found the concept fascinating and it was explained in some detail.
The heat pipe was manufactured from a 300mm length of ordinary 22 mm copper pipe that was bent to about 80 degrees and sealed at both ends. When mounted in the heater one end of the pipe was vertical and was located inside the water tank, the lower part extended through the floor bent to about 80 degrees and finned to collect heat from the gas burner underneath.
Inside the heat pipe all air had been excluded by boiling some distilled water and then sealing the pipe. As school children we probably all watched the collapsing tin can experiment where a sealed tin full of steam is allowed to cool, and as the steam condenses back to water, the internal pressure drops, and the pressure differential from outside to inside increases until te overcome the strength of the tin and it collapses.
In the case of the Cascade, the pipe does not collapse, so the effect is a considerable vacuum is formed inside the tube.
The internal low pressure has the effect of reducing the water's boiling point, and in practice it was usually below 40C!.
The finned lower leg was the lowest point of the pipe, so the liquid water would always collect there, but the rest of tube was filled with water vapour, and the same temperature and pressure. But once the burner was lit, the water in the pipe would very quickly start to boil, releasing more vapour which raises the pressure inside the pipe. The warmer vapour would rise to wards the top of the tube, and when it came into contact with a cooler part of the pipe which was inside teh water tank it would give up its heat to the wall of the pipe and condense back to its liquid state, where gravity would return it to the finned section. The control system would turn off the burner when the tank of water was hot enough.
The fact this system starts to operate at such low temperatures would allow it to be used for teh towel. The rail's bottom section only needs to protrude into the CH water stream to be able to effectively pick up heat and transfer it all around the towel rail.