Thursday, 20 February 2014

Potential perils of plastic pipe

A college tutor recently suggested to me that the current environment in the heating industry does not encourage thinking.   Maybe I should therefore be less amazed when I come across plumbers who think copper and plastic pipe are interchangeable. 

Environmental considerations aside, a more direct issue is that of flow rate and pressure drops.  Bore size of plastic is considerably smaller than copper, and it seems that this is often overlooked.  I am not against plastic, but if installers select plastic pipe instead of copper without checking the size correctly, this could have a negative effect on heat pump’s COP.  

(If you drop-off ½ way through this - read the scenario at the end )

With respect to the materials of plastic and copper, to my surprise I found a very informative document that discusses primary energy of these two materials in some detail, and concludes that the total energy involved in mining and manufacturing copper is far greater than the total energy (including the crude oil) to manufacture plastic pipe.  Looking at other potentially less biased general data on copper and plastic, it seems to confirm that more oil is used processing copper than would be used to produce plastic.  

Another factor to consider, that mainly affects pipe runs to hot taps etc, is the heat capacity of the pipe material. Plastic has a relatively high specific heat, and the wall is thicker, but it’s light. The net thermal capacity of the two is fairly similar.
(The cold feel of copper is more to do with conductivity from the hand than heat capacity)

However, obvious factors aside, one of the biggest issues that could affect installations involving heat pumps relates to the internal bore diameter.  This could have a noticeable effect on the energy-efficiency of the system.

All metric pipes are measured by their outside diameter. As can be seen, with common pipe sizes (outside diameters), equivalent plastic pipes have considerably smaller internal area to copper. This has a dramatic effect on flow characteristics.

 The graphs below illustrate the relative internal dimensions of common pipes.

 (see AECB Water Standard)

A brief note about smoothness - It seems a common belief that plastic is ‘smoother’ than copper, but the inner wall ‘smoothess’ of the two is the same.  However, plastic can be one-piece with slow sweep bends. This is certainly ‘smoother’ than copper with tight elbows.   Re inner surface, we can assume the two materials are the same.

Whilst it is fairly easy to look-up the pressure drop resulting from a specific flow rate with a specific pipe, we can see from the 2nd graph at-a-glance the relative flow capacity since the cross sectional area loosely indicates flow capacity.

If a certain flow rate is required, then we can look-up the required pressure that is required across the pipe length (beginning of pipe to end of pipe length).  The internal bore must be chosen such that the circulation pump is not overly large and energy-wasteful.

This graph shows the pressure required to maintain a certain flow rate for a fixed length of pipe.  As can be seen, the pressure drop along the example pipe varies very dramatically, so the wall thickness makes very big difference. 
In this example, we can see that a 15mm copper pipe could be used with a common central heating pump (shown at 3.6m head, 36kPa). However, if plastic were chosen, then one would need almost 9m head to achieve the required flow - far beyond the capability of normal circulators. On the other hand, if 22mm plastic were chosen, the pressure requirements would be only 1m head (10kPa) which is likely to achieve very low circulation pump energy.

In real life, we tend to have a pump connected to a pipe system, and the flow rate that results is dictate by the balance between the pressure produced by the pump and the ‘restriction’ of the entire pipe work circuit.

For our final graph, we consider a pipe circulating with a fixed-pressure electronic pump (Alpha etc)

The above graphs show relative changes in flow rate that would result from a fixed pressure. If the pipe chosen were too small, then a larger circulation pump may be needed in an attempt to compensate for the extra restriction caused by the small internal bore.

The point here is that by choosing plastic instead of copper of the same nominal size, the system could potentially suffer unless the sizing is checked. There is of course no problem using plastic if it’s the right diameter.  Indeed, 28mm plastic may be an ideal choice for the connections from a heat pump simply to minimise noise transmission.  The best final solution is often a mixture of both plastic and copper for a multitude of reasons.

All this emphasises how dramatically the wall-thickness affects flow rates and pump pressure requirements, but how does this translate into reduced COPs?

Scenario (based on something I observed on a barn conversion)

Let us consider a radiator at a far distance from the heat pump.  The flow-rate relates to the pressure drop, which relates directly to the pipe-run length, and of course, the required flow-rate relates to the room size (bigger the heat demand, the more flow required).  In this instance the room is large.
The default pipe size choice would be normal 15mm (outside diameter), but if the sums are done, it may become apparent that the choice should be between either 15mm copper or 22mm plastic. 

How could a pipe with too small bore affect the COP? 

Radiators should be balanced, in general by throttling valves (lockshield)  on smaller radiators, and those with shorter pipe runs. However, this is actually quite a difficult thing to achieve with a heat pump because the temperature difference (water inlet to water outlet) may only be 5 degrees (°C).  (It’s much easier to measure and adjust when the temperature differences are larger).

It is not ideal, or easy, to have to throttle most other radiators on a system, and there is the added risk of the circulation pump not being big enough to cope.
The likely result of any ‘restrictive’ pipe-runs would be a reduced flow rate to the radiator.  This would result in a considerable area (the bottom) of the radiator being cool, and a reduced heat output to that room.
The obvious action to redress the short-fall of heat would be to increase the flow temperature by increasing the heating curve setting of the heat pump. i.e. increasing the flow temperate from say 40 to 45°C.    Now the heat pump has to heat ALL water to a 5 degrees higher level.  This is likely to reduce the COP by 10 to 12%.

This all indicates that one must never assume copper and plastic are interchangeable without considering the pressure drops and diameters.  That aside, we have also highlighted the importance radiator balancing.  Ideally the pipe runs to radiators would be laid out so that the flow rates are naturally about right without the need for much valve adjustment.  A little extra work on the design makes life much easier thereafter.

Anyone intersting in further reading on water flows - this site is very intersting