Lay Out The Array
Many older systems have had problematic histories due to improper array layouts. Thermal expansion in some systems has caused damage to the piping, while excessive flow rates have led to erosion of the piping in others. A properly laid out array is one that brings the performance of each collector in the array to or above design conditions while maintaining the physical integrity of the fluid circuit. There are a few key areas to pay attention to in laying out the array:
- Keep flow velocities below 5 fps. This means no row longer than eight 4 x 10′ collectors with 1″ headers or twelve collectors with 1.5″ headers.
- Allow for thermal expansion within the array. You should always allow for some ‘swing’ in the joints between the header for the row and the supply and return piping. If you pin the joint then something will fail.
- Plumb the rows in a reverse-return manner or allow some other method of balancing flow within the array.
- Insulate all lines and protect from UV damage with either a latex coating or jacketing.
- Consider installing isolation valves that allow any row to be brought down for service while the remaining rows are allowed to function.
Open Loop Array
The diagram on the left shows a typical array of 24 collectors in 3 rows of 8. This would be a standard layout for collectors with 1″ headers. There are ball valves on the supply and return lines that allow each row to be isolated. To relieve pressure in an isolated row, a pressure relief valve is located on the lower header that can also be used to drain the row. On the outlet of each row is an automatic air vent that allow trapped air to be purged. The final item on the outlet is a thermal bleed valve (i.e. dole valve) that opens near freezing to purge warmer mains water through the row to prevent freeze damage. The thermal bleed valve is the second line of freeze protection in case pumped recirculation fails.
Closed-Loop Glycol Array
The closed loop glycol array looks just like an open loop array with the exception that the thermal bleed valve has been omitted. Also, the air vent in closed loop systems may be a manual coin vent as opposed to the auto air vents of open-loop designs. This is possible because there is only a finite amount of air trapped in closed-loop systems that is released after the array warms up. This small amount of air can be released at one time after startup and then the array can be manually sealed.
The schematic of the drainback system is simplest of all as it needs neither a thermal bleed valve or an air vent. Also, there is no need for isolation valves on the outlet of each row because the loop is unpressurized and there is no way for fluid to flow back up into the collectors against gravity. The only valve in the system is an isolation/balancing valve on the inlet to every row.
The simplicity of components in the drainback system is offset by the requirement to slope all piping to drain. For proper draining, all piping should be sloped at a minimum of 1/4″ per linear foot. Special care should be taken with the location and selection of all valves and fittings so that they do not restrict the ability of the array to drain under gravity.
Balancing Flow Within Collector Rows
Not only is it important to balance flows between the rows of a commercial array, but it is often necessary to balance the flow within the row itself. The figure on the left shows the results of a study done in 1970 that looked at flow non uniformities in long collector rows. This figure shows the differences in absorber temperatures across a row of 12 collectors at low, medium and high flow rates. Quite surprisingly, there is a 20 C (40 F) temperature difference along the row. This temperature difference directly correlates to the amount of fluid going through the array, and it shows that the ends of the array have lower temperatures corresponding to high flow rates while the center is starved of flow. The effect of these flow non-uniformities is an overall decrease in output of the array row that can be very significant. Although non-uniformities are less with 8 collector rows shown in this section, it is usually beneficial to attempt balancing the row.
Two Different Methods of Balancing Flows Within Collector Rows
The first method of balancing flow is to use ball valves in the upper and lower headers as indicated above. The ball valves can be partially closed to equalize the pressure difference across the upper and lower headers. An IR gun can be used to measure cover temperatures as an indicator of flow distribution. When the array is balanced, the cover temperatures should be nearly equal. Care should be taken to ensure that restrictions in the valves do not compromise the ability of drainback systems to drain.
The other method of balancing flow within the array is to use a parallel-series arrangement where the flow is put through the first 4 collectors in the row in parallel, piped into a downtube, and then plumbed through the next 4 collectors. By limiting the number of collectors paralleled together, flow non uniformities can be nearly eliminated. Such an arrangement does of course double the flow rate going through each collector and thereby raises the pressure drop. Pressure drops however are typically minimal to begin with so the increase is tolerable. Such an arrangement can obviously not be used with drainback systems but works well in open-loop and glycol configurations