Protecting Water Heating Systems with Expansion Tanks: Page 2 of 3

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Water Heating Expansion Tank
Water Heating Expansion Tank
A solar expansion tank
A solar expansion tank (solar xtank; circled) is a critical component of a solar water heating system. The solar xtank pictured is sized to maintain system pressure during stagnation.
Pressure-relief valve
Manufacturers use a variety of pressure-relief valves for their pump stations. The rating for the valve (circled) in this PAW pump station is 87 psi.
Floor-mounted expansion tank
Some residential SWH systems may require the use of a larger, floor-mounted expansion tank (circled) to accommodate the steam expansion that occurs during stagnation.
A prevessel can protect the membrane from excessive temperatures
In systems with a large volume of HTF, a prevessel can protect the membrane from excessive temperatures by decreasing the temperature of the HTF in the expansion system.
Water Heating Expansion Tank
A solar expansion tank
Pressure-relief valve
Floor-mounted expansion tank
A prevessel can protect the membrane from excessive temperatures

When stagnation occurs, the fluid temperature in flat-plate collectors can reach temperatures of roughly 350°F. In evacuated-tube collectors, the fluid temperature can exceed 400°F. In most antifreeze systems, these temperatures will cause steam to form in the collector array. When water turns to steam, its volume increases by a factor of 1,700. As a result, a teaspoon of water that turns to steam will force out the total fluid volume of two standard-sized flat plate collectors.

This fluid must go somewhere, otherwise the system pressure will increase and the pressure-relief valve will open, releasing fluid from the system. If this occurs, glycol will need to be added to the system and the system will need to be repressurized to ensure proper operation. But an appropriately sized and properly located solar expansion tank can maintain system pressures and avoid activating the relief valve.

Solar expansion tank location. To handle stagnation, the expansion tank must be placed where it can prevent the expelled fluid from pushing more fluid into the collectors and generating more steam. By locating the solar expansion tank between the check valve and the collector inlet, glycol can be pushed out of the collector inlet and outlet to the solar expansion tank, and the steam will remain at the top of the system. If the check valve is located between the expansion tank and the collector inlet, the fluid is only able to exit the collector array through the collector outlet, which then forces more fluid into the collectors.

It is also important to install the solar expansion tank where its bladder will be protected from exposure to steam. The bladders of solar expansion tanks are typically rated for maximum temperatures of 200°F to 210°F and can fail if exposed to higher temperatures. If the piping between the solar expansion tank and the collectors is short, then a prevessel (aka cooling vessel)—a small tank installed on the piping stem leading to the solar expansion tank—may be required. The prevessel holds enough room-temperature glycol solution to offset that which would be forced into the solar expansion tank if steam forms in the collectors, and protects the solar expansion tank from excessive temperatures. But for most domestic SWH systems, 50 feet of 3/4-inch piping will contain enough fluid to avoid needing a prevessel.

If the solar expansion tank is able to maintain adequate system pressures during a steam event, the system should return to normal once the temperature in the collector decreases and the steam condenses back into liquid. Using a high-temperature propylene glycol will help prevent the heat-transfer fluid from degrading.

Solar expansion tank sizing. By sizing the solar expansion tank to accommodate steam formation in the system, some system designers are able to eliminate the need for any type of heat dump or other overheating protection. Thus, taking some time to familiarize yourself with the proper solar expansion tank selection can reduce these costs and simplify the system (and its installation), as well as reduce the system’s maintenance requirements.

To size a solar expansion tank so that system pressure stays below the pressure-relief valve’s rating, you must calculate the maximum amount of fluid that could be forced into the solar expansion tank. For most systems, the worst-case scenario occurs when the system stagnates due to reaching the solar storage tank’s high-temperature limit. At this point, all of the glycol in the system has reached its maximum thermal expansion, but stagnation has not yet occurred.

The glycol’s expansion (prior to steam formation) can be estimated by multiplying the glycol solution’s coefficient of expansion by the change in temperature. If the initial fluid temperature in the system was 60°F and the fluid is expected to boil at 240°F, then the glycol can be expected to increase in volume by 7%. For a system with a fluid capacity of 8 gallons, this would be an increase in volume of 0.6 gallon (see “Thermal Expansion of Glycol” table, previous page).

The collectors’ fluid capacity is the amount of fluid that is expelled from the collectors when steam forms. For certain heat-pipe-style evacuated-tube and serpentine-style flat-plate collectors, this volume may be as low as 0.3 gallons per collector. For individual harp-style flat-plate collectors, this volume may exceed 1 gallon.

Comments (7)

Tom Lane's picture

There seems to be some confusion about steam back versus boil back . I have never heard the term boil back used . There is a major difference and lack of understanding between evaporation , boiling , and steaming . BOILING REFERS TO APPROACHING THE TEMPERATURE AND PRESSURE TO CREATE STEAM OR the rapid vaporization of a liquid that is heated to the boiling point - the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental PRESSURE . A PHASE CHANGE from water / gycol to STEAM requires + 972 BTU PER POUND OF WATER . BOILING REQUIRES only +180 BTU per pound per degree rise to the PHASE CHANGE of steam . Evaporation will change water / glycol to vapor below 212 F , ESPECIALLY IN UNPRESSURIZED DRAINBACK SYSTEMS . Many solar " educators " do not understand that THE WATER /GLYCOL film evaporates out well before the residual film can be degraded . ALL pressurized indirect glycol solutions ARE DEGRADED AT TEMPERATURES OVER 212 F that have changed to steam .As a side note , many manufacturers ( AET for example ) void their warranty in drainback systems if deionized , RO , or distilled water is used but will honor the warranty IF GLYCOL IS ADDED AT 30 % OR HIGHER to distilled , RO , OR deionized . A oversight in many Solar Books and Guides . Tom Lane

dave@atomicsolar.biz's picture

No solar water heater should ever stagnate, and I'm sory some designers produce systems that do under normal or power loss conditions. This leads to glycol degradation, and excessive pressure swings, and adding a large expansion tank may allow you to prevent the pressure relief from operating, it does nothing but save the boiling glycol from escaping, which indicates a problem, but is no longer apparent. If no problem is apparent, the glycol will turn acidic and eat the collector tubing, or lose it's freeze protection and burst. This is bad for the industry. Use DC pumps, PV, and properly sized expansion tanks for pressurized glycol systems, and heat dumps on larger space heating systems.

Vaughan Woodruff's picture

Dave,

Thanks for your comment.

Proper design can mitigate the concerns of acidic glycol due to overheating. As you are likely aware, the type of glycol makes a difference. Using glycol solutions with properly-rated inhibitors goes a long way.

The concept of "steam back" or "boilback" - when done correctly - can also minimize wear and tear on the glycol. If the system is only able to produce a fixed amount of steam when stagnation occurs (i.e. the steam stays trapped at the top of the system and no additional fluid is pushed into the collectors due to proper check valve location) and the collectors empty well, then the glycol really isn't exposed to excessive temperatures. The collectors empty immediately when a small amount of steam forms in the collector at the boiling temperature of the fluid (roughly 220-235F depending upon the system pressure) and as a result the glycol never sees temperatures above this.

A DC motor powered by PV has its issues as well - limited control options, the occasional need for overheating protection to protect the tank, limitations with certain systems, the potential for future shading of the PV module to cause issues. As with any SWH system, it really comes down to choosing the right approach for the application. Many are finding that the use of a "boilback" method with certain antifreeze systems is a robust solution that avoids the problems you mention. Others prefer PV with a DC pump.

Best,

Vaughan

Tom Lane's picture

I am glad to finally see HOMEPOWER Finally putting the expansion tank with the inlet facing up AND before the pump . It should be mentioned that it should be before the pump by a factor of 12 . That being 12 tines the diameter of the pipe . A tragic mistake in Home Power has been to show constantly the use of swing y or t check valves or even spring loaded check valves . As has been well documented in the American Literature and European literature now - these checks valves result in massive thermosyhon ing of heat back to the roof at night . At a recent IAMPO convention in Philadelphia this was discussed in detail . There are many examples from Bristol Stickey and other sources of exterior heat exchangers frozen and ruptured using mechanical check valves . It has be well documented by the Florida Solar Energy Center that only electrical solenoid check valves wired to the pump should be used on open loop direct or closed loop indirect systems to stop thermosyhon ing . IN POINT OF FACT A 50% glycol mixture will thermopyphon faster and easier than water . The steam back method of heat dissipation is degrading to the glycol solution as the glycol must go through a phase change to steam to Steam Back and not just vaporize out . A heat dissipation system or a vacation mode should be used in the control system . No American manufacturer I.e. AET , SunEarth , etc warrants their collectors to be used in steam back systems . The Europeans Schuco who are now out of the solar hot water business introduced this in to the USA with serpentine collectors . It is critical to understand that unless you especially design a interior bottom of the tank heat exchanger , that only electrical solenoid valves should be used as check valves . Solar Hot Water Lessons Learned 1977 to Today is an excellent book to learn about how to properly do glycol systems .

Vaughan Woodruff's picture

Tom,

Your statement about SunEarth is incorrect. If you go to the "Sizing Expansion Tanks" page on their website (http://sunearthinc.com/design-resou...) you will see that they specifically state that "the acceptance volume must be sufficient to accommodate expansion of the heat transfer fluid when the solar loop goes into stagnation."

Any antifreeze system with a differential controller is susceptible to stagnation. This is simply one way to mitigate the issue.

Vaughan

Tom Lane's picture

Vaughn , my statement about SunEarth had nothing to do about the article you posted from their website about expansion tanks excepting the expansion from stagnation . No American Manufacturer covers collector corrosion due to glycol fluids becoming acidic . The proper term Steam Back refers to a European design to empty the collector during stagnation WITH SERPENTINE COLLECTORS OR OTHER SPECIAL DESIGNS OF THE ABSORBER in the collector which NO AMERICAN MANUFACTURER CURRENTLY MAKES . SHUCO was the only company promoting Steamback in the USA and have long since left the American and European Thermal market place . If you call SunEarth , AET , or Solar Skies - the three largest American manfacturers or Heliodyne You will see that they do not approve Steam Back systems or have them in any of their designs . My biggest problem with Steamback systems is they require mechanical check valves , which are notorious for allowing reverse thermosyhoning , which requires special internal bottom of the tank heat exchangers to prevent overnight heat lost . I am glad to see Home Power magazine addressing these issues , however , very little has been written about reverse night time thermosyhoning with non electric check valves . Bristol Stickey has recently written about this in Plumbing magazines ( including showing busted exterior heat exchangers ) and old issues of Solar Engineering and Contracting from the 1980's throughly covered this problem , which is also a major problem for open loop direct systems in non freezing climates . Tom

Tom Lane's picture

Vaughn , my statement about SunEarth had nothing to do about the article you posted from their website about expansion tanks excepting the expansion from stagnation . No American Manufacturer covers collector corrosion due to glycol fluids becoming acidic . The proper term Steam Back refers to a European design to empty the collector during stagnation WITH SERPENTINE COLLECTORS OR OTHER SPECIAL DESIGNS OF THE ABSORBER in the collector which NO AMERICAN MANUFACTURER CURRENTLY MAKES . SHUCO was the only company promoting Steamback in the USA and have long since left the American and European Thermal market place . If you call SunEarth , AET , or Solar Skies - the three largest American manfacturers or Heliodyne You will see that they do not approve Steam Back systems or have them in any of their designs . My biggest problem with Steamback systems is they require mechanical check valves , which are notorious for allowing reverse thermosyhoning , which requires special internal bottom of the tank heat exchangers to prevent overnight heat lost . I am glad to see Home Power magazine addressing these issues , however , very little has been written about reverse night time thermosyhoning with non electric check valves . Bristol Stickey has recently written about this in Plumbing magazines ( including showing busted exterior heat exchangers ) and old issues of Solar Engineering and Contracting from the 1980's throughly covered this problem , which is also a major problem for open loop direct systems in non freezing climates . Tom

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