|
Article Excerpt
In the Dec. 2008 Solar Design Notebook column, we calculated the temperature that the absorber plate in a solar collector could reach under stagnation conditions. If stagnation occurs on a hot/bright summer afternoon, that temperature could exceed 350[degrees]F, and it's very likely the fluid in the collector will vaporize. This has to be taken into account when sizing an expansion tank for the collector circuit. With the proper sizing it's possible for the collector array to stagnate without causing the circuit's pressure relief valve to open.
[FIGURE 1 OMITTED]
Figure 1 shows the relationship between absolute pressure and the boiling point for a 40% solution of propylene glycol, a common collector circuit fluid.
To maintain this solution as a liquid at a stagnation temperature of 350[degrees]F requires an absolute pressure of about 118 psi (with a corresponding gauge pressure of about 103 psi) in the solar collectors. This is not practical, and may even violate some mechanical codes that require the collector circuit pressure to be no higher than the pressure of the domestic water.
Thus, we can't count on pressurization to suppress boiling under stagnation conditions. Under these conditions, the liquid in the piping leading to and from the collector array could also be very hot.
To prevent the relief valve from opening under these conditions, the expansion tank must absorb both the fluid displaced by vaporization in the collectors, as well as the expanding volume of the remaining liquid in the circuit.
Selecting the PRV
Before calculating the volume of the expansion tank, it's necessary to know the rated opening pressure of the system's pressure relief valve....
|
|
Looking for additional articles?
Search our database of over 3 million articles.
Looking for more in-depth information on this industry?
Search our complete database of Industry & Market reports by text, subject, publication
name or publication date.
About Goliath
Whether you're looking for sales prospects, competitive information, company
analysis or best practices in managing your organization,
Goliath can help you meet your business needs.
Our extensive business information databases empower business
professionals with both the breadth and depth of credible,
authoritative information they need to support their business
goals. Whether it be strategic planning, sales prospecting,
company research or defining management best practices -
Goliath is your leading source for accurate information.
|
|
