By David G. Palmer, Ph.D.
Summary: Preventative maintenance is always a good thing and, in the case of distillers, reducing the amount of scale buildup on the inside of boiler tanks can save time and money on service calls and extend the life of the equipment.
Distillation is a very sufficient efficient way to deal with inorganic and microbial contaminants in drinking water. A traditional distiller can also deal effectively with volatile organic compounds (VOCs) when used in conjunction with a granular activated carbon (GAC) pre- or postfilter. A distiller that incorporates forced condenser ventilation (see “Distillation: Forced Condenser Ventillation—A New Approach to VOCs,” WC&P, February 2000, pp. 106) in its design can cope with volatile organic contaminants without the need for GAC filtration. Such an appliance is arguably the perfect water purification device.
Meeting the enemy head-on
Hard water is the enemy of a distiller’s boiling tank. When such water is heated, bicarbonate ions start changing into carbonate ions and carbon dioxide gas is given off:
2 HCO3– ——–> CO32- + H2O + CO2 (g)
The solution rapidly becomes supersaturated with calcium and carbonate ions that precipitate out on the internal surfaces of the boiling tank, including the heating element, as a hard material (scale). Simply heating triggers scale formation. Frequent draining of the boiling tank only marginally reduces scaling. Too frequent draining will enhance scale formation.
Scale adversely affects the boiling tank operation. In particular, it acts as an insulator around the heating element, forcing the internal temperature of the element higher and higher. Ultimately, the element wires burn out and the element fails. By this time, the boiling tank contains large volumes of scale that can only be removed by prolonged soaking—typically overnight—with an acid solution. With countertop distillers—where the boiling tank is inspected when water is added—scale build up is highly visible and a weak acid solution, such as citric, can be used to clean the boiling chamber. Where visual inspection of the boiling tank isn’t part of normal operation, it rarely needs regular cleaning regular cleaning is rarely practiced by the owner. Cleaning requires removal of the unit at a service center, or a minimum of two service calls to allow for soaking. Replacing the heating element isn’t always easy when heavily encrusted with scale.
Scaling to a minimum
When hard water is softened by conventional ion exchange methods, calcium and magnesium ions are replaced by sodium. Sodium compounds are typically much more soluble than their calcium and magnesium counterparts. When softened water is heated, the same reactions occur with respect to bicarbonate ions changing into carbonate ions. However, because sodium carbonate is extremely soluble, supersaturation is slow to occur with most ions staying in solution. Almost no scale is formed on the heating element or internal surfaces of the tank.
When a distiller is placed in a building where the water is being softened, scale formation will be dramatically reduced. Normal water softeners do a good job in reducing hardness to a low level. Actual hardness of the treated water at any instant will vary depending upon recharge settings and also the volume and initial hardness of water passing through the bed at that time. Both high volumes and very low volumes may lead to detectable hardness in the water drawn by a distiller. The former arises because of insufficient contact time with the exchange media. The latter is due to leakage and will be more prevalent when the softener is regenerated with brine flow in the direction of normal service flow as opposed to counter current regreneration. Hence, even when a domestic softener is present, scale can build up with time inside the boiling tank.
Exchange softener cartridges
Distillers are efficient users of water with almost all the influent water being converted to high quality drinking water. It’s therefore feasible to install a portable cartridge containing conventional softening resin in the water line before the distiller. This provides almost zero grain hardness feed water, and the boiling tank will remain virtually scale free. The use of a suitably sized softening cartridge minimizes the need for boiling tank maintenance. It’s worth installing even when the feed water is passing through a conventional water softener. This ensures zero grain hardness water irrespective of the water softener operation, or any other water demand present while the boiling tank is replenished.
With time, the resin in the cartridge will approach exhaustion and the hardness of the water to the boiling tank will increase. It’s desirable to exchange the cartridge before this occurs.
When to change
With knowledge of the resin type and volume in the cartridge, hardness of the feed water and daily consumption of water from the distiller, it’s possible to make a conservative estimate of the time between changes. As a rule of thumb, allow a demand based upon 0.5 gallon per person per day. Time between exchanges for various hardness levels and family sizes is shown in Table 1.
Some newer distillers have a meter, which records the number of hours the heating element operates. As most distillers use approximately 3.0 kWH per gallon, the volume of water having passed through the softening cartridge can be estimated using the following formula:
(Hours run) x element power in kW
Gallons used = ——————————————-/3.0
More sophisticated controllers, using microelectronics, allow hardness and softener capacity to be set in the field. The microprocessor calculates the number of hours of operation before the softener needs to be exchanged and alerts the user when that time arrives. It can even be programmed to show the phone number to be called for local service.
When the service call is made, it takes but a few minutes to swap out the spent cartridge with one that has been recharged back at a service center. Time spent performing maintenance inside the consumer’s residence is kept to an absolute minimum. And the high cost and inconvenience to the user of a burned-out element is avoided.
Cartridge types
For a cartridge to be portable, there are limits on weight and size. Conversely, an exchange capacity as high as possible is desired. For areas with hardness below 10 gpg, a cartridge with a capacity of 3000 grains processes up to 300 gallons before needing to be exchanged—a total of 600-person days. A larger softener is more suited for higher hardness water and/or higher daily demand for drinking water.
Examples of exchange cartridges are shown in Fig 1.
Recharging
Any cartridge will last longer first time around. This is because it’s impossible to totally reverse the ion exchange process with current recharge procedures. Domestic water softeners aim to achieve the best salt efficiency and operate with lower recharge efficiency. They simply recharge more often. With a stand-alone cartridge the need is to maximize the resin capacity, which determines the time between service calls. Thus, the approach to recharging is quite different. With proper flow rates and brine concentrations, the exchange capacity can be returned to above 80 percent of new resin capacity. (The 6,750-grain capacity softener will have an actual capacity of around 8,000 grains first time around. The nominal 6, 750 grains reflects the capacity that can be achieved with proper recharging.)
One advantage of the exchange system is that fouling of the resin by iron—or the hardness minerals themselves—poses less of a problem. Such fouling can be removed at the service center using the controlled addition of a resin cleaner to the brining solution or a soak in resin cleaner before brining. The cost of chemicals to recharge a cartridge is less than $1.00.
Conclusion
It’s much quicker and easier to exchange a softening cartridge than it is to descale a distiller boiling tank. A service agreement with a distiller owner, based upon the use of a softening cartridge, provides advantages to both the owner and the dealer:
- Minimum time is spent on the consumer’s premises;
- Because it’s preventative in nature, the timing of a service call can be scheduled in advance;
- The user’s supply of drinking water is not interrupted; and
- Having to exchange the cartridge on a timely basis is, in itself, justification for the need for a service agreement.
About the author
Dr. David G. Palmer is president of Palmer Technologies Inc. (PTI), a specialist in distillation technology based in Lincoln, Neb. A native of New Zealand, Palmer came to the United States in 1985 to work for Pure Water Inc., a manufacturer of small distillers. He launched PTI in 1992. He holds a master’s degree in physics from the University of Otago, New Zealand, and a doctorate in physical chemistry from the University of London, in England. Palmer can be reached at (402) 474-7171 (phone/fax) or email: [email protected]