Technical Talk

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99% EFFICIENT DIRECT CONTACT WATER HEATING
FOR SERVICE WATER APPLICATIONS

B. Keith Walker

WHAT IS DIRECT CONTACT HEATING

(click on image for full size view)

Direct contact water heating is a heat transfer method in which there are no tubes isolating hot combustion gasses from the fluid to be heated. The exhaust gases are allowed to come into direct contact with water in a non-pressurized environment that is open to the atmosphere.

Direct contact water heaters consist of a vertical hollow chamber in which water is sprayed at the top of this vertical chamber. The upper portion of this vertical chamber is filled with stainless steel balls (or other shapes) that provide a large heat transfer surface area for the heat of the rising exhaust gasses to be conducted into the water. This "heat transfer zone" is approximately 24" to 36" deep and almost all heat transfer in a direct contact water heater occurs in this area.

In the same vertical column, below the "heat transfer zone" a burner is mounted which provides the energy used to heat the water. Depending on manufacturer, this burner may be substantially removed from the falling water or it may reside directly in the path of the water flow. The burner is forced draft and is typically fired on natural gas or propane although fuel oils can be used some of the time.

ULTRA HIGH EFFICIENCY

Direct contact water heaters are very energy efficient. In all burning devices water is a by-product of the burning process, because oxygen and hydrogen from the air combine to make H₂O. This water is vaporized immediately from liquid to gaseous form in the heat of firing. This phase change of water from liquid to gas is a cooling process and accounts for about a 12% thermal efficiency loss. Direct contact water heaters super cool the combustion gasses below the point where this newly formed water vapor will re-condense back into liquid form. By returning the water back to a liquid state direct contact water heaters are able to reclaim all of the thermal energy that is normally lost out the stack of a traditional boiler. The phase change that steals energy by changing from liquid to gas returns heat energy when the gas is returned back into liquid form. By transferring heat in this way direct contact water heaters are capable of achieving efficiencies that approach 100%. Typical fuel savings over a boiler installed at an existing facility are 30% - 40%.

NON PRESSURIZED VESSELS

Direct contact water heaters operate at a pressure that is only a couple of inches water column above atmospheric pressures. What this means is that all water that is sent into a direct contact water tower loses all of the water pressure that was present in the cold water feed lines before the water entered the heater.

Because the water loses all pressure there must be a pool for water to collect in after passing through a direct contact heater. This is done by having a tank or holding area at the base of the vertical heating tower. Water collects in this tank and is then held until the hot water is required at a remote location.

Mechanical pumps must be used to re-pressurize the water in this tank and move the volume required to the needed application. These pumps can provide virtually any pressure that a customer might require and can be used as a pressure booster over the typical water pressure that a facility would see off of a regular city water line.

Since direct contact water heaters operate at ultra low pressure this also means that the vessels are not pressure vessels. Because of this the heaters are inherently safer and can be used to reduce facility insurance costs over operating a pressurized boiler vessel.

WATER QUALITY

Since water comes into direct contact with hot combustion gasses the natural question is what happens to the quality of the water after having been passed through a direct contact water heater. I do not have data for competitive products, but I do have independent laboratory results for the QuikWater heater. This heater has a dedicated firing chamber that allows fuels to be fully combusted before the exhaust gases enter the water stream in the vertical tower and are not representative for any other brand of direct contact water heaters. The testing that was performed used city municipal water from the city of Sand Springs Oklahoma as the fresh water supply. Water samples were taken for this water both before and after having been passed through a QuikWater heater. The synopses of the test results are as shown in the adjacent table:

The results from the testing show that the QuikWater direct contact water heater has no appreciable negative affect on water passed through it and actually improves water quality slightly by removing volatile and semi-volatile chemicals that occur naturally when water is treated with chlorine at city water treatment plants. In other words if potable water is brought into the QuikWater direct contact heater the water will still be potable after having been heated.

(*** Acknowledged by the analytical laboratory as a probable laboratory contaminant.)

EXHAUST STACK EMISSIONS

Exhaust emissions from direct contact water heaters are typically favorable. NOx Emissions from direct contact water heaters are lower than boilers with the same Btu input (and the same burner). CO emissions are also low and are not a problem for direct contact heaters. Ultra low emission heater packages are available from other direct contact heater companies, but all data given here is for the QuikWater heater product. The burner used is the Maxon Kinedizer mounted onto QuikWater’s Patented dry firing chamber. The burner typically achieves a usable 4 or 5 to 1 turndown ratio (depending on heater model) with excellent CO and NOx emission numbers. The values shown are for a 8,000,000 Btu/Hr output heater that was tested for pre-certification submission in Southern California. Each firing position was fired three times and data was collected for 30 minutes for each of the twelve runs made. The results are as follows with only the highest values given:

Similar results are achievable for other QuikWater model sizes. Several heater sizes have a maximum NOx emission below 20 PPM. Flue gas recirculation is not used because the exhaust stream of a direct contact water heater is saturated with moisture. While flue gas recirculation achieves the emission results there are longevity problems with the blower/burner packages due to the extra moisture contained in the exhaust stream.

WATER TEMPERATURE LIMITATIONS

Direct contact water heaters have a difficulty in achieving outlet water temperatures higher than 180 ° F. Direct contact water heaters with dry firing chambers can achieve water temperatures up to 190 ° F. However, to achieve higher temperatures can be difficult to sustain in certain altitudes and conditions. Direct contact water heaters operate with a system pressure just a couple of inches water column above atmospheric pressure. Because of this lack of pressure, one of the characteristics of direct contact water heaters is that as heat is added to a liquid water droplet the droplet reaches a temperature where the liquid water will phase change into a gaseous form at a lower temperature than in a pressurized system. As talked about earlier a change from liquid to gas steals heat energy. In a non pressurized environment the cooling effect of the evaporation fights against the desire of a customer to heat water above 180 ° F. This water vapor that is created is typically re-condensed so the heat energy is not typically lost.

APPLICATIONS FOR DIRECT CONTACT

Direct contact water heaters can be used in almost any application that has a need for large (sometimes not so large) quantities of hot water. For my own purposes I have made three general classifications of water usage. These groups are:

1. Single Pass Application

2. Open Loop Recirculation Systems

3. Special Applications

A short list of applications has been given below. There are many more possible applications, but the list is:

SINGLE PASS APPLICATIONS

Single pass applications are the easiest and the best applications for direct contact water heaters. In these applications water is simply passed through a heater and then the water is consumed and not returned back to the heater. In these applications the efficiency of the direct contact heater is at its highest, the rate of calcium build up is slowest, and system maintenance requirements are low.

OPEN LOOP APPLICATIONS

Open loop applications are also easy applications for a direct contact water heater. In these applications the water used in the customers system is being recirculated, however water is still consumed either by washers, sinks, evaporation, carry off, Etc. In these applications the efficiency of direct contact heaters can still be in the 97 – 99% area, the calcium build up rate is a little faster than pure consumption applications, but the calcification rate is still very slow, and maintenance is usually low. Attention does need to be paid to the maximum flow rate requirements of a loop. Since the maximum and minimum flow rates are dramatically different, the physical size of the heater, atmospheric storage tank capacity, and/or the number and size of transfer pumps may need to be increased to accommodate the maximum flow requirements.

SPECIAL APPLICATIONS

Special applications are those processes that need to heat caustic or hazardous liquids, true closed loop systems, systems that include full body human contact, and non standard fuels (standard fuels are natural gas, propane, and number 2 fuel oil).

It is not a question that these applications can not be done by direct contact water heaters. It is only a statement that there are some special requirements either in the construction of the heater or special piping to avoid certain problems that can arise. Efficiencies in these applications are still good. There may be some calcification in the heaters that will force more frequent preventive maintenance schedules or water softening that would not otherwise be needed. Some of the applications may need to purge water periodically or chemically buffer for pH control. Overall, direct contact is suitable for the special applications, but certain engineering and economic factors should be considered to avoid surprises.

Direct Contact water heating presents a unique and reliable solution to large volume water heating, while presenting a very "green" solution to energy and environmental issues. Best of all, it dramatically reduces operating costs and increases profits

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