Modern boiler plants are increasingly focused on energy efficiency, reducing their physical footprint, as well as minimizing capital and installation costs. Companies are striving to keep their total cost of operation as low as possible to remain competitive in the global economy. One of the challenges is having a steam supply that can handle large variations in demand. If the boiler is too small to handle peak demands, the plant cannot operate as designed. If the boiler is sized to handle short bursts of steam, the boiler may be oversized and have higher energy input versus output due to lower efficiencies at lower loads.
Boiler efficiency can be impacted significantly when the boiler is oversized for various reasons. Combustion is much more efficient when operated at higher firing rates. When burners are continually cycling from high fire to low fire or even turning off, efficiency is lost. During the restart of the burner, the combustion air fan must be run without lighting the burner to purge out any fuel due to leakage. During shutdown, any remaining fuel must be burned off from the combustion chamber as well. Both of these waste fuel and energy. Boilers also lose energy from radiant heat losses from their shell. At lower firing rates, the percentage of energy lost is greater than at higher firing rates, again lowering the efficiency. All of these stops and starts also cause wear on the equipment, increasing maintenance costs.
One of the main causes of these wide operational variations is the batch process. These are particularly difficult because they require large amounts of steam in bursts which can be either long or short in duration. Industries such as laundries, expanded foam, canning, and brewing may be excellent candidates. When steam demand peaks, a pressure drop may occur causing potentially serious problems. As the pressure lowers, it may cause rapid boiling of the water shutting the boiler off due to low water levels. As boiler water is added, it could cause the steam blanket to completely collapse shutting the plant down due to no steam. Another scenario could allow the pressure to drop but still producing wet steam. This wet steam has many aspects that are detrimental to operations:
The question then becomes, “How do you satisfy the short periods of high demand without compromising efficiencies?” The answer may very well be as simple as installing a steam accumulator. These units have been around for years but are often overlooked during system design. These vessels act as a steam storage system that can release steam when demand is greater than the boiler’s production capacity and to receive steam when the demand is lower than what the boilers are producing. This allows for a smaller boiler than peak demand requires and prevents oversizing the boiler and operating at lower efficiencies. Steam accumulators can be very responsive to demand swings, lowering capital and installation costs while taking up a much lower physical footprint.
A steam accumulator is an insulated steel pressure tank containing hot water and steam under pressure that can be released when demand is higher than the capacity of the boiler system. They allow a plant with a low load demand to inject surplus steam into a large amount of water which is under pressure. Over time, the stored water increases in temperature and pressure until it achieves the saturation temperature for the operating pressure of the boiler. If the tank has more steam/water than needed, a trap removes excess water/condensate to maintain the accumulator water level and sends the condensate back to the feedwater tank. This allows room for the higher volume steam when the water flashes to steam. When demand exceeds the capacity of the boiler, the resulting pressure drop inside the steam accumulator will cause some of the hot water to flash into steam. The steam is then used to meet the demand upon the system. The accumulator is recharged during the next period of time in which there is surplus steam due to demand which is lower than the boiler systems output.
While a steam accumulator may seem like the ultimate solution for many applications, there are some important considerations that must be reviewed. There must be sufficient excess capacity during lower demand cycles to fully charge the accumulator. The total steam demand must also be lower than the overall boiler capacity. The accumulator must be sized appropriately to be able to store enough pressurized hot water and allow for the steam expansion during peak steam demand.
Steam accumulators come in a variety of configurations, and one example is shown below.
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Written by: Jeff Lazor