A closer look at what really goes on inside a dispenser.

Figure 1. Post-Mix Dispensing System


Prior to 2000, the primary device used for backflow prevention on post-mix dispensing systems was the ASSE Standard 1032 backflow preventer. Although ASSE Standard 1022 was issued in 1996, the first devices that were certified to meet this standard were not listed until 1998 and 1999.

In 1999, the beverage industry began its evaluation of the new devices and, by early 2000, began using them on nearly all newly installed carbonators. The vast majority of carbonators built and installed in the United States since 2000 are equipped with an ASSE 1022 backflow preventer and numerous carbonators built prior to 2000 have been retrofit with the ASSE 1022 devices.

Figure 2. Types of Dispensers

How These Systems Work

To fully understand backflow prevention in a post-mix dispensing system one should have a basic understanding of how these systems work. A post-mix dispensing system is like a miniature bottling plant. They are used at several hundred thousand locations, such as restaurants, convenience stores, theaters, office cafeterias and numerous other locations to manufacture beverages where they are dispensed and consumed. The post-mix dispensing system requires that only the syrup (approximately 1/6 of the complete beverage) be transported to the location, greatly reducing the logistics and transportation costs. Because of this, post-mix systems rely on the municipal water supply to deliver high-quality potable water for the remaining 5/6 of the completed beverage.

A post-mix system can be relatively simple (as shown in Figure 1) or it can become quite complex when multiple dispensers, multiple carbonators, water filtration, water chillers, water distribution manifolds and water pressure boosters are added to the system.

The typical post-mix dispensing system includes: a cooling or refrigeration system, six to 12 dispensing valves, a source of compressed or liquid CO2 (carbon dioxide), gas pressure regulators to reduce and regulate the CO2 pressure, a carbonator to produce carbonated water (also known as soda or sparkling water) and a syrup system that includes at least one syrup (BIB) pump for each syrup flavor.

The cooling and dispensing functions of the system are usually combined in the post-mix dispenser. The three common types of dispensers that combine the cooling and dispensing function (Figure 2) include:

Ice-Beverage dispensers,

Drop-In dispensers, and

Counter Electric dispensers.

Ice-Beveragedispensers are typically used for consumer self-serve applications and are capable of dispensing both ice and soft drinks. In many instances, an ice maker is installed on top of these dispensers to supply ice for the dispensers’ cooling function and for use in the finished beverage.Drop-Indispensers are used primarily by trained crew members and dispense only beverages. Ice, if needed, must be manually scooped into the cup before dispensing the beverage.Counter Electricdispensers are used in both self-serve and crew service applications. Some Counter Electric dispensers will include an integral carbonator.

The Remote Refrigerationsystem is a fourth type of post-mix dispenser system. By the nature of its design and use, the cooling system remains separate from the dispensing tower. This system is very similar to the Counter Electric dispenser, which combines the dispensing valves with the cooling system. Remote Refrigeration systems are typically used in applications that require a large cooling system to meet heavy beverage demand. This necessitates the need to place the cooling system, which, in most cases, includes an integral carbonator, in a location remote from the dispensing tower. These systems may be located in a back room or basement.

The cooling systems of Ice-Beverage and Drop-In dispensers are built around an internal aluminum cold plate that is used for cooling soda, water and syrup. The cold plate is sealed into the bottom of an ice bin, which, when filled with ice, provides the cooling mechanism for the dispenser. Inside the cold plate, long stainless steel tubes are cast in aluminum to form individual circuits used for cooling water, carbonated water and each syrup flavor.

Cooling occurs as the products are transported through the individual stainless steel tubes. In all cases, water, carbonated water and each syrup flavor remain separated inside the individual stainless steel tubes until they are dispensed through one of the dispensing valves.

Counter Electric dispensers are built with an internal water tank, sometimes called a water bath, which is cooled by a self-contained mechanical refrigeration system. This refrigeration system includes a compressor, condenser and evaporator. The refrigeration system freezes a portion of the water in the water bath to provide ample cooling capacity for the dispenser.

Unlike the Ice-Beverage and Drop-In dispensers, the individual stainless steel tubes for water, carbonated water and each syrup flavor are immersed in the ice cold water bath of the Counter Electric dispenser instead of being cast in an aluminum cold plate. And like the Ice-Beverage and Drop-In dispensers, water, carbonated water and each syrup flavor remain separated inside of individual stainless steel tubes until they are dispensed through one of the dispensing valves.

In the typical post-mix system, CO2 gas is supplied to both the carbonator and the syrup pumps from the CO2 source. Pressure-rated reinforced plastic tubing is used to transport the CO2 gas to the gas pressure regulators, where the pressure is reduced to the operating pressure required by the post-mix system. The CO2 source can be either high-pressure bottles or bulk tanks.

The typical post-mix system uses one of the two regulators to supply CO2 to the syrup pumps to pump syrup from the syrup packages (BIB) to the dispenser at a preset pressure, usually between 60 and 80 psi. The second regulator supplies CO2 gas to the carbonator. In the carbonator, CO2 gas is used to manufacture carbonated water and to push the carbonated water to the dispenser, usually between 90 and 110 psi for an ambient carbonator or between 70 and 80 psi for a cold carbonator.

Carbonator with an ASSE 1022 backflow preventer. Courtesy of McCann's Engineering.

Carbonator Operation

The typical carbonator, shown in top photo, is composed of the following major components: a carbonator tank, a liquid level control, a pressure relief valve, a CO2 check valve, an ASSE 1022 Backflow Preventer, a high- pressure water pump, a pump motor and the interconnecting plumbing. In most cases, the carbonator water pump is brass, with an integral 100 mesh strainer with an open area of at least four times the cross-sectional area of the inlet pipe.

The strainer is located on the inlet side of the water pump to ensure that debris doesn’t enter the water pump, check valves or carbonator tank and is easily accessible for service and maintenance. In some remote cold carbonator systems, stainless steel pumps are used for pumping carbonated water.

The sole purpose of the carbonator is to produce carbonated water for the post-mix dispensing system to be used in the manufacture of carbonated beverages. It is in the carbonator tank where potable water from the municipal supply is mixed with CO2 to create carbonated water. When the carbonated water level in the carbonator tank drops below the low water level (as shown inFigure 3), the liquid level control starts the pump motor. The motor drives the water pump, which pumps water through the backflow preventer into the carbonator tank. As water is pumped into the carbonator tank, which contains an atmosphere of CO2 gas, it creates a mechanical action that mixes water with CO2 gas to help speed the absorption of CO2 to form carbonated water.

The typical water pump is capable of delivering water at a pressure sufficient to overcome the internal carbonator tank pressures and still be able to either atomize or mix the water with CO2 gas to promote fast absorption of CO2. Once the liquid level control senses that water has reached the high or full water level (as shown inFigure 4), the pump is stopped. Note that both water and CO2 gas enter the carbonator tank from the top while carbonated water is taken from the bottom of the tank by means of a stainless steel dip tube.

Figure 3. CAD drawing showing a carbonator at a low water level.

Although the primary objective of the carbonator’s backflow preventer is to ensure that neither CO2 nor carbonated water backflow into the municipal water system, it is also essential that the backflow preventer be installed between the water pump and the carbonator tank to ensure the proper operation of the carbonator. This also prevents carbonated water from contacting the brass water pump. The backflow preventer is needed in this location because the internal pressure in the carbonator tank can exceed the municipal water pressure by 100 psi or more. The water pump is used to boost the water pressure to overcome this large pressure differential, and the backflow preventer prevents the backflow of carbonated water from the carbonator tank when the pump stops.

Currently the soft drink industry uses ambient and cold carbonators. Ambient carbonators are the most common type of carbonator in use today. Cold carbonators, which have become more prominent during the past 8 to 10 years, are composed of all of the same components as the ambient carbonator, but the carbonator tank is located within the post-mix dispenser separate from the rest of the carbonator system. Other than the location of the carbonator tank, the only other difference is that water is chilled prior to entering the carbonator tank.

Figure 4. CAD drawing showing a carbonator at a full water level.

The ASSE Standard 1022 Backflow Preventer

The ASSE Standard 1022 backflow preventer is a vented, dual-check valve composed of an atmospheric vent port located between two independently acting check valves that are biased in a normally closed position. Its design provides for a fail-safe device. When there is no flow through the backflow preventer, both the primary and secondary check valves remain closed. These check valves prevent the backflow of carbonated water and CO2 gas.

The secondary check valve is located on the water supply side and the primary check valve is located on the carbonated water side, and, in combination, they serve to separate the higher-pressure CO2 gas and carbonated water from the water supply. When the carbonator starts a refill cycle, usually after a dispensing valve is opened and water flow is initiated downstream from the backflow preventer, both the secondary and primary check valves open to allow water to flow through the device.

Under normal operating conditions, during static or no flow conditions, water pressure on the inlet side of the secondary check valve holds the diaphragm of the secondary check valve against the atmospheric vent seat to prevent water from leaking through the vent port, and the secondary check valve remains closed. During this time, the pressure between the primary and secondary check valves is approximately 2 to 4 psi lower than the inlet water pressure.

If the pressure between the primary and secondary check valves equals or exceeds the pressure from the municipal water supply, the diaphragm on the secondary check valve will lift off of the atmospheric vent seat, permitting flow through the atmospheric vent port. In this situation the secondary check valve remains closed, preventing backflow into the municipal water supply. This is what happens when there is a backflow condition or if the primary check valve fails to completely seal.

The media leaking past the primary check valve (CO2 gas and/or carbonated water) is exhausted through the atmospheric vent port, lowering the pressure between the primary and secondary check valves to approximately 14.7 psi (atmospheric pressure at sea level), thereby preventing the leaking media from entering the municipal water supply. In most installations, the vent port is connected to a sight tube that is used as a visible indication of any discharge from the device. Leakage of water or carbonated water from the vent port provides a clear and visible indication of a check valve failure.

Before receiving the ASSE Standard 1022 seal, the device must pass the rigorous testing described in the ASSE Standard 1022 code by an approved testing agency, certifying and attesting that the device has been tested, inspected and that it meets the requirements of the standard. Testing by the approved agency includes endurance cycling of 500,000 cycles. Post-mix equipment manufacturers subject the devices to additional testing to ensure that they meet their requirements for reliability and suitability for the application.

There are currently several devices certified as meeting the requirements of ASSE Standard 1022 and each is marked accordingly with the ASSE 1022 seal. The devices predominantly used in the manufacture of post-mix dispensing equipment are the ABCO (Anderson Brass Company) model AB-1 and Watts Regulator model SD-3. The Conbraco model 4C-100 series CBBP was the first device certified to meet ASSE Standard 1022.

Since its adoption by the beverage industry, ASSE Standard 1022 has been incorporated into the FDA Food Code and is referenced in NSF Standard 18 as an acceptable form of backflow protection for use on post-mix dispensing systems. Currently every major plumbing code (the Uniform Plumbing Code, the National Standard Plumbing Code and the International Plumbing Code) requires either an internal or external backflow preventer that meets ASSE Standard 1022 to be installed on soft drink fountain dispensers to prevent carbonation backflow. The new code language allows use of the ASSE 1022 device to meet the respective codes without the need to install a separate backflow device on the waterline to the dispenser.