Why do we preheat the combustion of air?
The main objective of preheating the air is to enhance the thermal efficiency of heating processes. In boilers, preheated air recovers its heat from the boiler’s flue gas. This leads to the improved boiler thermal efficiency brought about by the lessened level of heat loss in the boiler’s gas.
What is combustion air temperature?
The primary air is heated up to about 170°C by means of a heat exchanger with the exhaust gas. The secondary air is the postcombustion air injected into the upper part of the combustion chamber and is necessary for completing the combustion process.
What is combustion air preheater?
An air preheater is any device designed to heat air before another process (for example, combustion in a boiler With the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.
What does combustion air mean?
Combustion Air – (1) Air that is supplied to combustion appliances to be used in the combustion of fuels and the process of venting combustion gases. Inadequate combustion air can lead to dangerous problems. (2) The duct work installed to bring fresh, outside air to the furnace and/or hot water heater.
What is the effect of heating the air with the preheater?
As air was preheated, the flame temperature increases accordingly, due to the fact that the heated air increases the energy input of the flame, leading to higher combustion temperature.
How much combustion air does a furnace need?
A furnace must be supplied a specific amount of fresh air per cubic foot of gas burned. To burn 1 cubic foot of gas, approximately 1,000 Btu for natural gas, requires 10 cubic feet of air for perfect combustion (Figure 2).
How much air is required for combustion?
Therefore the quantity of oxygen required for combustion of 1 kg of the fuel is: (2.66C + 8H + S) kg. ∴ Oxygen required from air for the complete combustion of fuel will be (2.66C + 8H + S – O) which can be written as 2.66C + 8 (H – O/8) + S, the term in the bracket being known as the available hydrogen.
How much air is needed for combustion?
For natural gas-fired burners, the stoichiometric air required is 9.4-11 ft.3 / 1.0 ft. of natural gas or approximately an air-to-gas ratio of approximately 10:1. In this case, there is an excess oxygen level of 2%. In the combustion zone, it is challenging to measure excess air.
Why do we reheat steam?
Why Reheating of steam is used? Explanation: Work output = work done by turbine – work done by compressor. Explanation: Since work output increases the steam rate decreases. Explanation: Efficiency = work output / heat supplied, heat supplied is less in this condition only.
How does air temperature affect combustion?
High air temperature accelerates the rate of combustion and results in a high flue gas temperature. This increase in flue gas temperature has two effects. First, increasing the temperature under fuel-rich conditions drives the reactions (as shown in Figure 7) toward the formation of N2 rather than fuel NO.
How much combustion air is needed?
Do I need a combustion air?
It is the air necessary for a gas appliance such as a water heater to function properly. Not to get too basic here, but gas appliances require a certain amount of air to be used for combustion, much the same way that the fire in your fireplace needs air to get started and to keep burning.
Does pre-heating combustion air increase NOx levels?
Pre-heating combustion air does have the effect of increasing NOx levels, because of the higher flame temperature. This higher value still remains well below maximum allowable level, in the range of 80 ppm. Finally, duct run must be carefully designed, if both make-up and combustion air travel together in the same duct.
How does preheating improve the efficiency of fuel-fired industrial heating?
For fuel-fired industrial heating processes, one of the most potent ways to improve efficiency and productivity is to preheat the combustion air going to the burners. The source of this heat energy is the exhaust gas stream, which leaves the process at elevated temperatures.
How do you preheat the air in a gas boiler?
The most common way to preheat the air is with a heat exchanger on the flue exhaust. The heat exchanger can be either air-to-air or air-to-liquid-to-air. Combustion air pre-heater technology on large coal-fired boilers dates back to the early 1900’s, and is used on most all large boilers todays.
What is the cost of combustion air heating?
For a system requiring two 10-MMBTUH units, the differential cost of material between conventional and pre-heated combustion air can be summarized as follows (in US dollars): With a nominal natural gas cost of US$3.50/MMBtu, a 2-MMBtu/h heat recovery represents 7$/h savings, or $60 000 per year — or a 10-month payback.