In the last few months, requests for Spray Dryer Heat Recovery Systems have increased a lot. These systems pre-heat the spray dryer inlet air using waste heat, most often from the spray dryer exhaust. As more companies are working to meet their ESG (Environmental, Social and Governance) goals, it’s clear that a spray dryer is often the largest energy consumer or carbon producer on the factory site.
Incorporating Spray Dryer Heat Recovery into a new planned spray dryer is the easiest. Retrofitting heat recovery is often much more complex, but still achievable.
In my experience over the last 30 years, I’ve seen that although there is an ROI on retrofitting heat recovery, it usually takes 7-12 years to achieve. There always seems to be other site projects and priorities with a better ROI that take precedence.
Interest in heat recovery always increases when energy costs rise, but stainless steel and general construction costs always seem to rise in lockstep. The number of years necessary to achieve the ROI always seems to remain the same. When stainless steel and construction costs drop, energy costs have dropped too. An appetite for the implementation of Spray Dryer Heat Recovery requires the added corporate incentive that comes with having setting environmental targets associated with ESG goals.
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Before getting into the types of heat recovery systems and technology, I want to take a moment to discuss the impact and potential of using the spray dryer exhaust to preheat the incoming air. Heat recovery has the most impact and potential when the spray dryer exhaust temperature is high.
When a spray dryer does single stage drying (all drying in the main chamber, no fluid beds), the exhaust temperature is typically 190F to 220F depending on the product. This yields an attractive result because there is a significant differential temperature to the incoming air, which might be 10F-40F in winter depending on your location and 80-90F in summer. This allows the reasonable recovery of 12-18% of the energy in the exhaust air and transfers it to the incoming air. If the spray dryer in question has Indirect Gas Heat (90% efficient), then the savings might be 1-2% higher.
When a spray dryer does multistage drying and the chamber exhaust air is mixed with the exhaust from fluid beds, the overall exhaust temperature might be 145F-175F. This yields a lower heat recovery of 8-14% depending on the specifics of the dryer and products being dried. If the chamber exhaust air is segregated and not mixed with the fluid bed air in the exhaust, then the heat recovery potential is more like that described for single stage drying.
The heat source doesn’t necessarily need to be the dryer exhaust, especially if the economics does not provide an ROI for that scenario. The incoming spray dryer air could be pre-heated with other sources of waste heat.
Spray dryer heat recovery generally is premised on having a fin-and-tube coil in the inlet air. This is typically a copper tube and aluminum fin coil, unless stainless steel is required for cleanability. The heat transfer liquid moves through this coil. Cu/Al coils have the best heat transfer. Because the air velocity across the coil face is the same as the air velocity across the filter face, these coils are typically installed right after the main inlet filters.
As outlined above, the heat from the dryer exhaust can be captured in a heating medium (glycol solution) and transferred to this coil at the inlet, but any waste heat source is a possibility for pre-heating the dryer inlet. If there is an evaporator that produces COW Water and that COW Water isn’t used for other heat recovery (milk preheating), then this can be used to pre-heat the spray dryer air. If there is a surface condenser with an evaporator that goes to a cooling tower, this water can be used to pre-heat the spray dryer. Any waste heat source where the liquid is heated to 130F or above, and is in enough volume, can be used to pre-heat the air for the spray dryer.
Most typically, it is the exhaust air of the spray dryer that pre-heats the incoming air for the spray dryer. There are several types of heat exchangers that can be used for the exhaust air, but commonly a fin-and-tube coil is used.
A fin-and-tube coil presumes:
a) the exhaust duct and supply duct are far enough apart that an Air-to-Air Heat Recovery System is not possible due to the ductwork arrangement and cost; and
b) there is a bagfilter before the coil to prevent significant powder from collecting on the fins.
There are other types of tubular heat exchangers possible, especially when higher levels of powder are present. To allow effective cleaning, the coils are typically stainless steel tube and stainless steel fin. This material choice has a heat transfer cost and reduces the heat transfer rate.
The most expensive part in considering spray dryer exhaust air heat recovery is the duct transitions. The coil will have a large cross-sectional area to allow lower air velocities and thereby the whole unit will require a fair amount of space, either inside the building or on a new exterior platform, and will have large duct transitions.
A final type of spray dryer exhaust air heat recovery is an Air-to-Air Heat Recovery system. This is best for new dryers and is difficult to retro-fit because it’s premised on the idea that exhaust air and supply air must be physically near each other to cross through a heat exchanger. This leads to the best energy recovery, but often the equipment arrangement is not ideal, and the extra ductwork necessary to make the air streams cross makes the solution cost-prohibitive.
Process Note: If the spray dryer in question uses a direct-fired natural gas heater for heating the main drying air, pre-heating the air this way not only saves gas, but the water that is a product of the combustion is also removed from the air stream, allowing a slight but measurable increase in capacity. Depending on the value of the powder produced, this may also factor into the ROI equation.