Fan Speed Control for Crop Stores - Energy Saving?

In these days of high energy costs speed inverters have been promoted as potential energy saving devices. Our view is that in most instances they should come with a Government “HEALTH WARNING” for your stored crop.

The facts usually stated is that if you cut the speed of your fan by 50% you save 80% of your electricity running costs. What is not made clear is that running these fans at this speed will give little or no benefit to your crop whatsoever and a far cheaper option, (which involves no Capital Expenditure at all!) is to use the OFF switch which we guarantee will save you 100% of your electricity bill.

Fans selected for Crop Stores have been chosen to give an output air volume and pressure sufficient to ventilate the building area and stored tonnage involved.

Reducing the fan speed in a typical Box Store will reduce the air volume significantly and therefore result in poor air distribution and giving much longer running times, which will almost certainly de-hydrate the crop.

Much more seriously in a typical Bulk Store reducing the air volume will lower the fans pressure capability to get the air through the crop to a point where no useful effect at all will result. Reducing the fan speed to 50% decreases the fan pressure by 75%.

In refrigerated stores the design air volume is critical the achieve the design performance of the refrigeration system. If the air volume is reduced, the air on to off temperature differential is compromised, and the high humidity desired will be lowered.

Probably 90% of Crop Stores in agriculture are DX type. In these stores the air volume reduction could well lead to catastrophic compressor failure due to liquid refrigerant returning to the machine as a result of the reduced heat exchange from lack of air flow.

Only in the minority of Glycol water cooled stores can this be avoided but the compromised humidity and lack of air distribution is still undesirable to the stored crop.

Inverters are an expensive luxury. Consider this - a traditional Star Delta motor starter for a 15kw Fan Motor costs around £400. An Inverter Starter costs around £2,250, more than 5 times more. On the maintenance front most star delta starters will operate for 15 to 20 years without any service at all. In addition most local electricians can fault find and repair star delta starters in an emergency breakdown.

Inverters are modern electronics. An Inverter model is not normally produced for more than 5 years at most. Therefore if one fails after this period it is likely a new replacement is the only option. (£2,250 + Fitting) Our experience of inverters is that a life of less than 7-8 years is more than typical. More seriously, your local electrician is unlikely to have an inverter in stock and down times of days, if not weeks are quite possible.

The other worry for inverters is the effect on Fan Motors. Many fan motors are engineered to use the airflow over the motor to keep it cool. Reducing this flow for protracted periods could result in overheating and premature motor failure.

All this may cause you to think that Inverters are USELESS . Far from it, they have many very appropriate applications. Water pumping is good example were a reduced liquid flow is required. Even in Crop storage we use them to control the outside Condenser fans speed to maintain a system pressure balance.

The point we are trying to make is that in fan situations where the performance has been calculated to do a job it is counterproductive to reduce the fan speed.