Accounting for friction loss in solar loop piping is important to properly size a solar hot water system. Here at Metro Solar Atlanta we use this engineering principle to correctly size solar hot water systems.
With copper each increase in size also increases the cost. Not only does the piping itself cost more but the insulation costs more as well. So for that reason it makes sense to keep the piping down to the smallest diameter possible and still have enough flow to meet the demand of the system.
Let’s set up a hypothetical solar hot water installation.
Let’s say that the length of the run from the tank to the collector is about 60 feet one way. So that makes the entire loop roughly 120 feet plus the length of the piping through the collector and the heat exchanger. We won’t go into the friction loss of the collector or heat exchanger today, just the piping in the loop since it is usually the major contributor. (residential systems)
In order to figure out the amount of pressure drop due to friction in any pipe you can use the Hazen-Williams equation. The Hazen Williams equation is: Pd = 4.52 x Q^1.85 / (C^1.85 x d^4.865)
Pd = pressure drop
Q = flow in gallons per minute
d = average inside diameter
C = roughness constant (140 for copper)
Let’s start out by sizing for 1/2” pipe. The diameter of Type L 1/2”copper tubing is .545”.
Let’s use a hypothetical flow rate of 3 gpm.
Pd = 4.52 x 3^1.85/(140^1.85 x .545^4.865)
Pd = 8.45psi
This does not include any fittings the collector or the heat exchanger.
This much pressure loss is unacceptable. Friction loss should be kept as low as possible but should not ever exceed 5psi.
So let’s look at the same system using 3/4” piping to see if we can get this number down to a manageable size.
The diameter of Type L 3/4”copper tubing is .785.
Pd = 4.52 x 3^1.85/(140^1.85 x .785^4.865)
Pd = 1.4psi
So the pressure loss is 1.4psi. This number is a lot more manageable. We can live with this even if the fittings, collector and heat exchanger double this number we are still below 4 psi which is below our 5 psi rule of thumb.
Pump curves are labeled by “feet of head” or some other similar title. Pump curves show the relationship between the amount of head the pump can provide at a given flow rate. So we need to convert our pressure loss into feet of head. To do this we multiply the psi by 2.31.
In our example, we would multiply 1.4psi by 2.31 which gives us 3.23 feet of head. After adding in our losses due to fittings, collector and heat exchanger we can use the pump curve to select the proper size pump for our system. This gives us the ability to select a pump that meets our demand and uses the smallest amount of electricity to perform it’s duty.
There are many free online resources that will calculate friction loss for you.
Engineers Toolbox has many online tools.