When I was a junior evaporator operator in a small dairy plant in the Midwest, I ran a small Rogers 2-effect rising film skim milk evaporator and a box dryer that that was sized to match the evaporator capacity. This was back in the days when almost all the controls were truly manual.
I turned valve wheels to make the system change. To increase product flow, I tweaked a graduated control valve. To change the effects level, I tweaked another graduated control valve. To increase steam pressure to the 1st effect, I turned another handwheel. It really was a touchy-feely operation.
At the time, I had a supervisor that was constantly trying to get me to increase the evaporator concentrate solids up, so that it did not cost as much to run the spray dryer.
As an operator, I knew that as I increased the evaporator solids, the evaporator became more difficult to run. The thicker material within the evaporator caused it to foul out quicker, requiring a premature shut down and a longer CIP.
That supervisor and I had a lot of sometimes heated discussions about where to run those solids off that evaporator.
It’s about four decades later and I now know in so many ways my supervisor was right. Even that old (1950s vintage) evaporator was at least four and maybe five times more efficient removing water from the product than the dryer was. Depending on the evaporator type, modern evaporators generally remove water 4-15 times more efficiently than a dryer per dollar cost of energy.
The bottlenecks for most drying plants now are not the evaporator. Modern evaporators have computers and a high level of automation with lots of instrumentation that help us realize what is happening and how well the evaporators are running. We can usually push modern evaporators to at least 100% design evaporation with concentrates at design solids.
Dryers are limited by maximum air inlet temperatures, maximum airflow, ambient humidity and how much water they have to take out of their feed to make powder to specification.
Increasing dryer feed solids is an easy way to add capacity on the dryer and in turn, allow rate increases on the evaporator.
Bottom line: You make more product with less energy.
Let’s say we are bottlenecked by our skim milk dryer. We have it running maxed out at 5,000 lbs of water removal. We are feeding the dryer with feed that is 48% solids, 52% moisture.
That works out to a feed rate of 9,968 lbs/hr feed with 4,785 total solids and a powder rate of 4,968 lbs of powder at 3.7% moisture.
Let’s say we make some adjustments to the evaporator and can now achieve 51% solids to feed the dryer. We can now feed the dryer at 10,630 lbs/hr. This will give us a powder rate of 5,630 lbs/hr. while still removing the 5000 lbs/hr. of water with the dryer.
This can increase powder production by 661 lbs/hr. or about 13.3% without adding significant energy to the dryer and only slightly more energy to the evaporator.
Increase powder production by 13.3% and this equates to an extra 13,222 lbs. of product made per 20-hour day, about 396,673 pounds per 30-day month or about 4,627,850 pounds extra product made per year. The numbers get big in a hurry.
These kinds of capacity increases give plant management important options:
If they don’t have the product to run, then they could shut the system down for over 2 hours per day.
Or this capacity increase allows you to shut down one day per week to work on equipment, save energy or reduce staffing cost, yet produce the same amount of powder.