Volume 21, number 1, March 2007

Direct contact technology: Operation, applications and savings

Hot water boilers using direct contact heat transfer between combustion gases and water are recognized for their high energy efficiency. To achieve this full energy potential, it is essential to use them in a suitable application.

Operating principle

The heat transfer principle is simple (Figure 1). High-temperature combustion gases in the region of the flame become saturated with water by evaporation of descending water. This water vapour condenses again when it comes into contact with the stainless steel nodules, kept relatively cold by the water jets at the inlet. At the outlet of this nodule section, the combustion gases encounter an even colder environment as they pass under the cold shower. The combustion gases thus are cooler and exit the water heater at a temperature close to the water inlet temperature.

Figure 1: Direct contact water heater

The colder the water temperature at the water heater inlet, the colder the products of natural gas combustion will be when they emerge, and the closer they will propel the direct contact water heater’s thermal efficiency to 100%.

Given that the combustion gases come into direct contact with water, it is important to verify whether the low concentrations of contaminants coming from these gases have an effect on the process. If such is the case, it is possible to operate with a primary and secondary loop system. This prevents the process water from coming into contact with the water from the water heater. Given that this technology triggers condensation of combustion products, natural gas becomes the choice energy source. In some applications, these water heaters can reach a thermal efficiency of 99%.

Typical applications recommended
  • Hot process water production
  • Sanitary hot water production
  • Preheating of boiler make-up water
  • Cool air heating
  • Low-temperature hydronic heating system

Advantages compared to traditional boilers
  • Higher energy efficiency
  • Reduction of GHG emissions
  • Low maintenance needs
  • Rapid response to hot water demand
  • Low thermal loss in waiting period
  • Low thermal loss by radiation
  • Flexible regulation compared to pressure vessels.

However, it is very important during the design process to prevent a high water return temperature. If the water temperature at the water heater inlet exceeds the dew point of the combustion products (140° F), recovery of the latent energy contained in these combustion products will be reduced and will result in losses by evaporation. This ejected water vapour, containing about 1,000 Btu/lb, will be lost directly up in the water heater chimney and will reduce the product’s performance.

The graph shows the relationship between the inlet water temperature and the efficiency of the direct contact water heater.

Potential savings

The potential savings related to the use of this natural gas water heater are around 15% to 20% compared to a traditional steam boiler (see example).

Example:
a 100 BHP water tube boiler producing steam at 15 PSIG used in the production of hot water through a steam-to-water heat exchanger. If the return water temperature is 100° F, if the approximate thermal efficiency of this traditional boiler is 80% and if this system operates an average of 4,000 hours per year at 70% of its capacity, natural gas consumption will be approximately 336,000 m3 per year.
Based on the efficiency curve presented, the approximate efficiency of this same process with a direct contact water heater would be approximately 95% (water return at 100° F). The replacement of this boiler with a direct contact system would generate new consumption, which can be estimated as follows:
Under this example, about 53,000 m3 of natural gas is saved per year. As an indication, this saving would be reduced to about 20,000 m3 per year if the water heater return temperature were 140° F instead of 100° F.

Direct contact technology is very interesting to recover the thermal energy lost by existing equipment discharging hot gases, such as boilers and furnaces. The burner of a direct contact equipment can be replaced or combined with this hot gas flow to produce hot water, thus reducing the overall energy bill.

In short, the use of direct contact technology is simple and can generate very interesting savings. These savings can quickly amount to 15% or 20% if the technology is used in applications that assure a relatively cold water return.

Martin Blanchet, Eng.
Technical Adviser
DATECH Group