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 |
|
.jpg)
click for enlarged diagram
|
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 |
|
.jpg)
click for enlarged diagram
|
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. |
| Drainback
Array |
|
.jpg)
click for enlarged diagram
|
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 |
|
