Drainback Continued from page 46
tank temperatures seldom reach high limit due to the size of the load relative to the collector area. Closed loop sys- tems lend themselves well into these load profiles due to the simplicity of the design and installation. There is one important detail to pay attention to in either system, however — sizing the solar storage tank to
reservoir itself that are important to meet. One is that the volume of this reservoir must be larger than volume of collectors and piping above it. Another is that a piped vacuum break should be included at the top of the drain- back vessel so that during drainback, immediately fol- lowing the ceasing of circulation, air can be released from the top of the drainback reservoir to escape out of the tank and back into the collector array. A site glass should be included to view the water level in the drainback reser- voir. A flow meter may be substituted for a site glass if it is located at the level of fill. Variations of the drainback tank may include its loca-
Figure 1
the collector energy capacity. The goal to meet for a solar hot water system is to raise the temperature in the solar tank so that it first preheats cold water and climbs at or above design temperature (i.e. 120°F for DHW) while having enough storage volume to absorb the remain- ing daytime collection cycle. The general rule of thumb for solar storage sizing is that for every one square foot of collector aperture, you will need to store anywhere between 1 to 2½ gallons of water, depending on your location and the application. Large collector banks with small storage vessels can be problematic and should be avoided. The collector investment is wasted by too many idle hours during a daytime cycle due to excessive solar storage tempera- tures. The exceptions to that rule would be buildings, such as hospitals or process heat factories, with con- sistent daytime hot water loads.
Drainback designs with pressurized solar storage vessels There are a multitude of drainback designs which
could be covered in this article. Since the reader may be accustomed to using pressurized vessels, I have included a diagram for a basic drainback system that is comprised of a drainback reservoir, an external heat exchanger, and a pressurized solar storage tank (see Figure 2). I have also included a “how-to” design for drainback so the system will operate efficiently and trouble free. There are variations to the basic design that I will also cover. The most obvious criteria to be met for a drainback
system is that the top of the drainback tank must be below the bottom of the collector level. The collectors must drain into the drainback reservoir during non-collection periods. There are specifications about the drainback
Page 48/Plumbing Engineer
tion. It is not necessary to install the drainback tank in the mechanical room with the heat exchanger and the solar storage tank. In tall buildings where the mechanicals are in the basement, it is better to locate the drainback reser- voir high in the building. This choice is primarily made to size a pump for the collector loop lift that would not have to overcome the entire height of the building, saving on both pump size and operating cost. The drainback pump is also something important to pay attention to. The pump selection and installation are very different than those chosen for a closed loop system. Drainback pumps for drainback reservoirs are stainless steel or bronze construction. As fresh oxygen is usually present in a drainback tank, cast iron circulators would rust and fail. Always mount the pump below the static fill line as far as possible. In order to do this, the drainback vessel will sometimes need to be installed below the
Figure 2
drainback tank. For this reason, it is not unusual to see a drainback tank installed above ground level, and the pump located under the bottom of the drainback reservoir. This is not always the case, but for small drainback tanks under 20 gallons it usually is. The circulator must be capable of delivering the reservoir fluid at 2 feet per second through
Continued on page 50 March 2011
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