As concrete producers in colder climates renovate their ready-mix plants, they are finding that direct-contact water heaters provide ample hot water on demand — at lower cost than traditional direct-fired hot water tanks or boilers. Hot water is required to ensure consistency of the concrete mix during cold weather.
Direct-contact water heaters work differently from direct-fired water tanks with firetubes, and from boilers. Typically, they are vertical cylinders with a section of diffusion material inside the top; a burner in the middle; and a collection area at the bottom. Water is sprayed into the top of the heater, and it migrates downward through a section of diffusion material called packing. Packing typically is made from stainless steel formed into rings of various shapes and sizes. These rings provide the surface area where heat is transferred to the water from burner gases.
As the water flows downward through the packing, hot burner gases flow upward through the falling water. The heated water collects briefly in the bottom of the cylinder and is pumped directly from there to the mixing equipment.
Unlike a direct-fired tank with a firetube, with direct-contact water heating, there is no need to heat and store large quantities of water before it can be used. Direct-contact water heaters heat the water on demand. These types of heaters also take up less ground area than tanks. And, unlike boilers, direct-contact water heaters are not pressurized and require less maintenance.
One concrete producer recently installed its first direct-contact water heater at one of its concrete plants. The company determined that the direct-contact heater costs about half as much to operate as a direct-fired water tank at one of their similarly sized plants.
Generally speaking, direct-contact water heaters can be more efficient than direct-fired tanks with firetubes. There are a couple of reasons for this. First, their design allows a higher percentage of heat from the burner to be absorbed by the water. By contrast, fired boilers and direct-fired tanks have to maintain higher stack temperatures in order to prevent exhaust gases from condensing, which would cause rust and eventually interfere with burner operation. Higher stack temperatures mean more heat is being carried out of the heater. The direct-contact design readily allows water vapor in exhaust gases to condense and mix with the rest of the heated water.
Second, direct-contact water heaters do not have to maintain the temperature of several thousand gallons of water. Because water is being heated and used on demand, a standard burner is fired to heat only the amount of water as it is used. This reduces the amount of fuel used to heat the water.
Direct-contact water heaters heat the water on demand. These types of heaters also take up less ground area than tanks. And, unlike boilers, direct-contact water heaters are not pressurized and require less maintenance.
Direct-Contact Water Heater Maintenance
Direct-contact water heaters are low maintenance, with nearly all components easily accessed from the outside of the heater. Some units are designed to ease access to the heater’s packing material and combustion area. Quick-release hatches allow maintenance personnel to inspect and change out the packing. Manways or removable firing chamber heads provide access to the lower half of the heater. During troubleshooting, technicians can open the access hatches to inspect the packing. Then, if it needs cleaning, it can be taken out through the lower hatch, cleaned and put back in through the upper hatch.
Some plants use mix water from a well. Water is pumped from the well into a storage tank that feeds the direct-contact water heater. The water is heated to temperatures of 140 to 160°F (60 to 71°C) — or even higher — to overcome heat loss absorbed by cold aggregate and ambient temperature. This ensures that the mix can still be poured at optimum temperatures around 70°F (21°C).
Direct-Contact Water Heater Controls
Typically, mix temperature control is maintained through the heater’s control panel. It has a digital controller that allows plant personnel to enter the desired water temperature.
Many heaters are equipped with UL-rated, NEMA 4 control panels that house the heaters’ safety systems and PLC and digital controls. Compliance to UL standards helps ensure the operator of the quality of the electrical system and that the safety system meets current requirements for unattended operation. Low water-level switches prevent the burner from firing unless water is flowing through the heater. This prevents the walls of the heater from being damaged by overheating. High water-level switches shut off the burner and water flow to the heater. The NEMA 4 enclosure can provide watertight protection against splashing water, hose-directed water, rain, sleet, snow, ice, dirt and windblown dust.
One important aspect of having a successful installation is working together — end user and supplier — to establish the basic design parameters of the system, operating requirements and site-specific conditions. These include water, electrical and fuel supplies, and emission limitations.
Locations in ozone non-attainment areas need to carefully review the allowable NOX and carbon monoxide (CO) emission limits. Advances in ultra-low-NOX burner technology, specifically 100 percent premix burners, allow direct-contact water heaters to meet the most stringent, 9 ppm NOX emission limits with CO emissions below 50 ppm. Ultra-low-NOX premix burners generally are equipped with electronic, microprocessor-based parallel combustion controls to maintain set fuel/air ratios and ensure emissions are met over the operating range of the heater. Other NOX control methods involving flue-gas recirculation are impractical on direct-contact water heaters due to the high moisture content in the flue gases, which could impact flame stability and longevity of burner components.
In conclusion, like many companies looking for competitive advantages, ready-mix plants are always looking for new products that might help operations with reliability improvements or cost reductions. Direct-contact water heaters have a simple design that minimizes downtime, produces hot water on demand and offers fuel savings. Most designs use off-the-shelf components, so it is possible to go to a local burner supplier or electrical store to get parts and components.
Making sure the heater manufacturer has several full-time service technicians and an in-house parts department or local representatives will ensure support when necessary.
Direct-contact water heaters are vertical cylinders with a section of diffusion material inside the top, a burner in the middle and a collection area at the bottom. Water is sprayed into the top of the heater and flows down while hot burner gases flow up through the falling water. The water passes over the diffusion material, called packing, which provides the surface area to allow heat transfer to the water from the burner gases. The heated water collects in the bottom of the cylinder before being pumped to the mixing equipment.