A cement plant operates a Smiths ATOX50 vertical roller mill (VRM). Due to frequent air leakage during operation, the mill's efficiency has been significantly reduced. Upon learning that Senior Engineer Zhang often provides technical consulting services to enterprises, the plant sought his help to resolve the issue.
According to Engineer Mr. Zhang: A VRM system must operate under a high negative pressure, with strict control over external air ingress. However, achieving absolute airtightness is unrealistic in practice. Therefore, for the entire system – from the feed airlock valve to the exhaust fan after the dust collector – if the air leakage accounts for only 10%–15% of the total air volume, the system can still be considered effectively sealed. If leakage exceeds this range, corrective measures must be taken.
Common causes of air leakage in VRM systems:
1. Feed airlock valve without an inner liner – Over time, wear causes the inner surface to become uneven, leading to air leakage. Hence, the valve should be designed with a built-in inner liner.
2. Overflow material chute – Leakage prevention must be considered at the design stage. However, operators sometimes remove the flap valve used for sealing, claiming that fine powder in the overflow material occurs when no leakage exists. This phenomenon is actually caused by uneven upward air distribution around the grinding table – specifically, insufficient air volume on the side near the flap valve. The proper solution is to adjust the air guide ring (wind ring) resistance, rather than removing the flap valve to achieve a temporary balance.
3 Intentional cold air ingress for safety – Some operators intentionally allow cold air into the system to lower the mill's temperature and ensure safe operation. In such cases, operators must closely monitor the pressure difference between the inside and outside of the mill.
4. Cold air damper left open or not fully closed – The cold air damper installed at the hot air inlet to the mill is often found open or improperly closed, allowing cold air to enter. This reduces the inlet hot air temperature to only 200°C, impairing the mill's drying capacity and severely affecting kiln output and heat consumption. In this situation, the cold air damper should be completely shut.
