ig. 1. Modulating blower and gas valve assembly.

The Gas Technology Institute has developed a design for a fully modulating burner system that greatly improved the temperature control and performance of several residential/commercial appliances including a pizza style conveyor oven, a two rack bakery oven, a building humidifier and a steam generator. The burner system uses a premixed burner with a blower-gas valve assembly that varies gas flow rate based on the set point temperature of the appliance. The control system proportionally decreases the burner’s heat output as the appliance temperature approaches the set point, decreasing the range of temperature variation experienced by a single firing rate, on/off burner system.

Background

Fig. 2. Conveyor oven burner (top) and steam generator burner (below).

Traditional burner systems in foodservice appliances use a single firing rate burner with an on/off method of controlling the system temperatures. The burner fires until a designated control temperature reached a desired set point. The burner then shuts off and is not relighted until the control temperature falls below the set point. Depending on the appliance, this method offers respectable temperature control with a certain amount of temperature variation around the set point. The technologies that GTI used for its design of a conveyor oven and steam generator required a tighter control of the temperatures in the appliances to achieve the desired results. For comparison, appliances were tested using an on/off burner control system. However, significant improvements were observed when the appliances were operated using a modulating burner system. The baked products from the conveyor were more consistent in appearance and doneness and the steam production from the steam generator was easier to control and to provide a constant rate of steam.

Modulating system

The modulating burner system uses a combustion blower, gas valve, control system and burner. Different burners were used for the conveyor oven and steam generators and are described later. The burner system is a pre-mixed burner where air is pulled in by the blower and mixed with natural gas before entering the burner. (See Fig. 1.) The controller monitors a control temperature (return air in the conveyor oven and heat transfer fluid in the steam generator) and adjusts the blower speed to achieve modulation.

A change in air flow rate results in a change in the inlet pressure on the blower where the gas valve is mounted. The gas value responds to the change in pressure by adjusting the gas flow rate to the blower. The corresponding adjustment of the air and gas flow rates result in different burner firing rates, while maintaining the required air-to-fuel ratio for safe and effective burner operation. The firing rate is proportionally decreased as the control temperature approaches the set point.

Applications

Fig. 3. Commercial foodservice conveyor oven with modulating burner.

GTI incorporated the modulating burner system into three of its commercial appliance designs: a conveyor style pizza oven, a steam generator, and a building humidifier. Fig. 2 shows the conveyor oven burner (top) and the burner used in both the steam generator and building humidifier (below).

The conveyor style oven shown in Fig. 3 was designed with an airflow and combustion system that increased production rates while maintaining cooking quality. Several manufacturers market a conveyor style pizza oven for the foodservice industry. Despite the many different models on the market, the basic design does not vary greatly. Generally, a series of blowers provides heated air (from one burner) to a single conveyor deck in an oven. These ovens are usually designed for stacking in order to provide multiple decks. The modulating burner was used to improve control of the bake temperature compared to the same oven using a non-modulating burner.

The combustion system consists of burner blower and gas valve shown in Fig. 1 connected to a burner nozzle shown on the left in Fig. 2. This style of nozzle was used because the resulting flame is fairly tight and elongated for firing in the confined space in the oven.

The control system on the oven proportionally decreases the heat output from a maximum firing rate of about 210,000 BTU/hr as the oven temperature approaches the desired oven temperature set point. Within 1 Deg of set point, the burner modulates down to about 20 percent of maximum capacity.

The improvements in the temperature control of the oven are shown in Fig. 4 for the same oven operating with a modulating burner and a non-modulating or single firing rate on/off burner. The figure shows that the oven temperature varied over a range of 50 DegF for a burner that operates in on/off mode. The modulating burner reduces this range to less than 5 DegF. The oven has passed agency approval and is being marketed by AVANTEC.

Steam generator

Fig. 4. Results for modulating vs. non-modulating burner in the conveyor oven.

GTI also used a modulating burner system in a gas-fired steam generator for use in both building humidification and commercial kitchen applications. (See Fig. 5). Both systems generate steam using falling film heat transfer technology. This method uses a heat transfer fluid to heat a surface on which a thin liquid water film is established. The thin film of water evaporates without boiling (bubble formation) to limit particulate and/or droplet carryover. Because of the low water inventory, the steam generation rate is easily and quickly controlled.

An additional benefit is self-cleaning of the heat transfer surfaces since hard-water scale flakes off and falls to the bottom of the unit away from the heat transfer surface. The heat transfer fluid virtually eliminates burnout of the primary heat transfer surface because the heat transfer surfaces near the flame are not affected by scaling.

The burner system for the steam generator uses the same blower and gas value as the conveyor oven, but a different burner, shown on the right in Fig. 2. The cylindrical shape of the mess burner produced a short flame that was easy to contain within the heat transfer coil.

Fig. 5. Building humidifier and steam generator layout and system.

The control system proportionally decreases the heat output from the burner as the fluid temperature approaches the desired set point, typically 300 DegF. For a temperature difference of greater than 12 Deg, the burner fires at a maximum setting of about 210,000 BTU/hr. As the temperature of the fluid increases, the burner output modulates to a proportionally smaller firing rate until the fluid is within one degree of set point. At this point, the burner modulates down to about 20 percent of maximum capacity.

Comparison for the data using the steam generator showed that modulating the burner eliminated temperature swings in the heat transfer fluid, similar to the results for the pizza oven in Fig. 4. Variation of over 100 DegF were reduced to less than 5 DegF. By precisely controlling the fluid temperature, the steam output became much more consistent.

Conclusions

GTI has incorporated a gas-fired modulating burner system into its designs of three commercial appliances. Testing of the appliances showed that the variations in temperature of the working fluids (air or heat transfer fluid) were greatly reduced compared to a typical on/off or non-modulating burner. The improved temperature control produced a consistent baked product in terms of doneness and appearance in the conveyor oven and a constant output of steam in the steam generator and building humidifier.

Other benefits include less wear and tear on the system components due to the lack of over heating of internal components as the temperature varied around the set point and decreased system noise due the burner usually firing at a lower rate during operation. A non-modulating system fires at its maximum rate whenever the burner is on.