Most manufacturing plants have multiple central utility systems that support the plant operations. The more common systems are Steam, Chilled Water and Compressed Air. Go into any plant and there is a good chance you will see each of these utilities branching out to almost every area of the plant. In some cases, the distribution networks can extend for thousands of feet and the energy used to get the appropriate quantity of each utility to the desired locations can be substantial.
Typical manufacturing operations have inconsistent demand for each of the utilities. For a batch operation, large quantities of steam may be required for a short time at the beginning of the process to heat a batch, but after the batch temperature is reached little or no steam may be required for the remainder of the cycle. Likewise, cooling may not be required at the beginning of the batch, but a massive amount of cooling may be required when the batch is almost complete. Adding a degree of complexity, most plants have multiple operations that could be placing high demand simultaneously on the same utility.
Chilled water and compressed air utility systems have historically balanced the generation capability with some storage volume that allows for attenuation of peak demands. System designers have been comfortable using various technologies for cooling storage and routinely distribute compressed air storage throughout a facility to improve utility performance.
Steam has been another issue. It is commonly thought that a boiler must be sized for the maximum load because it is impossible or impractical to “store” steam. That creates a problem. Defining the maximum load in a dynamic manufacturing environment is only marginally easier that picking which team will win next year’s Super Bowl. The classical way is to add up all of the loads, apply a magical “diversity factor” multiplier to each load and then add a safety factor to the result. This is often considered an “engineered” solution. Unfortunately, the “diversity factors” are typically nothing more than guesses and the safety factors are applied in recognition that the entire design was uncertain to begin with. The result is a system that is over-designed for all but the peak usage times where it may be inadequate. Without any capability to store an appreciable quantity of steam, the operation of the system can be a challenge.
Too bad there isn’t a way to store steam? Actually there is a technology that has been around for over 100 years that does just that. It is called the “steam accumulator” and is essentially a vat of super-heated water held under pressure. When steam is required, a pressure-regulating valve is opened and steam flashes from the vat into the steam distribution header. In order for this to work, the pressure of the steam leaving the accumulator needs to be significantly less than the pressure of the steam entering the generator. For a very comprehensive discussion on the design of a steam accumulator, go to this spirax sarco page.
Now that you know this technology exists, the next question is probably “why don’t people use this?” Good question! Basic thermodynamics dictate that under optimum conditions you might be able to store 20 pounds of steam in 100 gallons of super-heated water. Not bad, but if you have a significant steam demand, the size of the accumulator would quickly become an issue. Large pressure vessels are expensive and take up a lot of room.
Here is where the twist come in. Instead of using an expensive pressure vessel, use a tubular heat exchanger. Fill the tube side of the heat exchanger with an appropriate phase change material that will melt below the charge temperature and solidify above the discharge temperature. Add a very basic pressure control system to the discharge of the heat exchanger and you have a very efficient steam accumulator that could potentially store 60 pounds or more of steam in that same 100 gallon volume (a sizable portion of the volume would be occupied with the tubes and thermal storage material).
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