Pump Tech Inc. Pump performance using a variable speed drive on a centrifugal pump has it&rsquo;s limitations which often are not recognized resulting in high capital equipment cost with very little, if any cost savings in operation.&nbsp; Usually, a centrifugal pump is specified to deliver a certain capacity at a fixed head (discharge pressure) and controlling the flow by way of a VFD can be an effective way to control flow BUT, the resultant head will be reduced by the square of the speed.&nbsp; Hydraulic fundamental rules are shown below :<br /> <br /> FLOW:&nbsp;&nbsp;&nbsp; Is directly proportional to speed<br /> HEAD:&nbsp;&nbsp;&nbsp; Is increased / decreased by the square of the speed<br /> BHP:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Is increased / decreased by the cube of the speed<br /> <br /> The following example is based on water at 70F, specific gravity 1.0<br /> <br /> Therefore, a pump specified to deliver 100GPM at 50FT&nbsp; at 3600RPM using 2.5BHP<br /> Would deliver&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;...&nbsp; 83GPM at 35FT at 3000RPM using 1.45BHP<br /> And&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;.66GPM at 22FT at 2400RPM using 0.74BHP<br /> And&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;&hellip;.50GPM at 12.5FT at 1800RPM using 0.313BHP<br /> <br /> Pumps are sized against a system resistance curve where increased flow will result in increased head requirements.&nbsp; But, as can be seen from the example, the head drops so rapidly that the pump MAY soon be unable to overcome the system resistance (particularly if the head involved is static elevation) and the flow would simply stop.<br /> <br /> In the writers opinion, VFD&rsquo;s do have their place in LARGE applications where fluctuating demand (ie: highrise apartment / office buildings) can be extreme during certain times of the day and near zero during the evening / nighttime hours. These installations typically have much larger HP motors and more frequent use where a power consumption saving can easily compound and offset the capital cost of the equipment.<br /> <br /> http://www.pumptech.com/Daves-Blog.page?ArticleID=35775 2010-09-09T06:15:23.8084747Z 9/9/2010 2:15:23 AM http://www.pumptech.com/ 9/9/2010 2:15:23 AM