The number of passes that the flue gas travels before exiting the boiler has been a good criterion when comparing boilers. As the flue gas travels through the boiler it cools, and therefore changes volume. Multiple pass boilers increase efficiency because the passes are designed to maximize flue gas velocities as the flue gas cools. ZOZEN has developed new design technologies in our WNS series boilers allowing for comparable efficiencies in fewer passes, resulting in smaller boiler systems that will fit in tighter quarters.
System load is measured in either BTUs or tons of steam (at a specific pressure and temperature). It would be nearly impossible to size and select a boiler(s) without knowing the system load requirements. Knowing the requirements leads to the following information:
The boiler(s) capacity, taken from the maximum system load requirement.
The boiler(s) turndown, taken from the minimum system load requirement.
Conditions for maximum efficiency, taken from the average system load requirement.
Determining the total system load requires an understanding of the type(s) of load in the system. There are three types of loads: heating, process, and combination.
Stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler. A well-designed boiler removes as much heat as possible from the combustion gases. Thus, lower stack temperature represents more effective heat transfer and lower heat loss up the stack. The stack temperature reflects the energy that did not transfer from the fuel to steam or hot water. Stack temperature is a visible indicator of boiler efficiency. Any time efficiency is guaranteed, predicted stack temperatures should be verified.
Stack loss is a measure of the amount of heat carried away by dry flue gases (unused heat) and the moisture loss (product of combustion), based on the fuel analysis of the specific fuel being used, moisture in the combustion air, etc.
In order to improve the combustion efficiency of the boiler, in a large boiler, the combustion air is not supplied all at once, but is supplied in two times, one is supplied with the fuel that controls the rate of combustion as well as the amount of fuel that can be burned,and the other is directly supplied to the combustion process that that improves combustion efficiency. The primary air rate of the boiler refers to the proportion of the primary air. If the total air volume is 100% and the primary air rate is 70%, the secondary air rate is 30%.
Economizer is one of steam boiler’s equipment which is used to heat feedwater before it is supplied into steam drum. Economizer is the heat exchanger equipment to increase boiler efficiency by absorbing heat recovery of flue gases. The lower temperature of flue gas out from stack, the heat loss will be less and the fuel which is needed to convert water into steam will be also less in certain circumstances. So it can be said that economizer can save the fuel efficiently. Economizer will make temperature of feedwater higher, so steam boiler can produce steam easily.
A dryback boiler has a rear wall that is lined with refractory, a wetback boiler has a rear wall that is jacketed by water. Because of this fact, wetback boilers typically boast higher efficiency than dryback boilers as the heat from combustion goes directly into heating water instead of refractory. Wetback boilers are also more forgiving with load changes as the reversal chamber of the boiler is totally submerged in water creating an even heat transfer on the intermediate tube sheet to furnace joint.
The boiler gas consumption calculation need the following parameters: gas calorific value and boiler thermal efficiency.
Theoretically, the gas consumption of boiler = boiler thermal capacity ÷ (calorific value of gas x boiler thermal efficiency )
Take the 1 tph steam boiler as an example:
= 600,000 cal / (8500Kcal * 0.98) =72m3/h, the 1 tph boiler's gas consumption per hour is about 72 cubic meters.
Steam boilers with a low NOx system are specially designed to minimize the environmental impact with the production of dry saturated steam. This type of boiler incorporates a stainless steel economizer with a cross flow integrated within the boiler and that, consequently, allows to reach maximum efficiency.In addition, it has a natural gas burner with low NOx emissions.
