Paper Mill Upgrades Air Dryer and Cooling System
This article reviews two major processes in paper mills: compressed air quality and air compressor cooling. The central air compressor room was expanded and relocated at the largest privately owned paper mill in Canada. The compressor space was required by a plant expansion, which would occupy the original compressor space for increased production.
The existing, open draft cooling towers for the centrifugal air compressors were aging, and required a major capital expense for relocation, repair & re-piping. An additional centrifugal air compressor was installed and a central 12,000 scfm refrigeration dryer was added to supply the entire plant with dry, oil-free air. The dryer was required to be a redundant, cycling dryer so that the mill could operate round the clock, while minimizing power consumption and the risk of downtime.
During the compressor & dryer pre-installation discussions, it became apparent that a better compressor cooling solution was possible, and within the constraints of the original project time frame. The mill pond water never exceeded 72F in the summer, but was not considered clean enough for the air compressors. The project proposed using the mill pond water with a plate & frame cooling system to supply clean water in a closed loop to the air compressors. This avoided the expense of tower refurbishing & relocation, while also eliminating the evaporation losses & make-up, along with the associated water treatment and maintenance. A strategic decision was made by the mill to change direction, and proceed with the new proposed closed-loop cooling system in parallel with the new compressor & dryer installation.
Large Capacity Dryer
The large capacity refrigerated dryer (12,000 scfm) was designed to satisfy the specific mill requirements for reliability, redundancy & energy efficiency. The shell & tube heat exchangers & integrated separator were designed for a maximum pressure drop of only 2.0 PSI at 12,000 SCFM. The maximum operating air compressor shaft BHP was around 2,500. Therefore a design pressure drop of 2.0 PSI was intended to reduce the air compressor power required by 50 kW/h compared to a traditional air dryer 5.0 PSI pressure drop.
Motivair Large-Capacity Cycling Refrigerated Dryer
The refrigeration was supplied by four semi-hermetic compressors with cylinder unloading, installed in four completely independent parallel refrigeration circuits. This design allows any circuit to be off-line for service or repair, while the others remain in full operation. Separate power disconnects for each compressor and glycol pump enable the operator to perform any maintenance safely, while the plant dry air supply is uninterrupted.
The dryer is a cycling design, which requires the refrigeration compressors to cool a circulating mass of glycol solution, which in turn cools the air in a separate heat exchanger. 2 separate glycol pumps, with auto change & alarm insure a continuous glycol flow. The refrigeration compressors sequentially unload, and cycle off, based on the applied compressed air load at any time. They automatically absorb only the minimum power required to meet the load. The PLC controller automatically rotates the compressor & pump sequence, and prevents short-cycling. The resulting dew point is a constant 33-39F, regardless of load changes.
The airborne condensate is separated from the air stream in a 2-stage ultra-high efficiency stainless steel separator which guarantees >99.9% separation efficiency from zero to 100% load. This is especially important in large capacity dryers, which can experience very low airflow between shifts, or in off-peak periods, weekends, etc.
The accumulated condensate is discharged via 3 x parallel pneumatically-powered drain valves. Any valve can be isolated and removed while the others remain in operation.
Since start-up in 2003, the dryer has never been inoperative, and has improved productivity at the mill through improved compressed air quality.
Compressor Cooling System
The mill’s air compressor cooling system was designed to cool the existing centrifugal air compressors and the new Motivair water-cooled 12,000 SCFM refrigeration air dryer, plus planned compressor growth for the next several years. The design heat load was established at 18,300,000 Btu/h, cooling 1,475 GPM from 110F to 85F, using 2,439 GPM of mill pond water at 72F, and leaving at 87F.
Compressor Cooling System
Motivair designed and built a single skid-mounted package, which consisted of a stainless steel plate & frame heat exchanger, duplex 75 HP circulation pumps with VFD, for the closed loop circuit, filtration of the mill pond water, and all associated 8” process control valves and piping in 316 stainless steel. The entire package measured 20 feet x 10 feet wide x 9 feet high and weighed approximately 20,000 Lbs. The cooling skid was easily installed inside the building, and the space occupied was a fraction of that required for the original outdoor cooling towers & pumps. The closed loop water temperature was comparable (85F) to an optimized evaporative cooling tower, but without the evaporation losses and water treatment required by a tower, in a smaller footprint, and at a competitive cost. The construction (all 316 stainless steel) was compatible with the typical airborne pollutants found in a paper mill.
The combined equipment cost of the Motivair 12,000 SCFM refrigeration dryer and the new closed loop cooling system was approximately US$350,000, plus installation. The entire cost will be recovered through increased productivity, by eliminating unscheduled downtime, and reduced system maintenance.
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