October 17, 2011
Getting Started with Cooling Towers
By Patrick Lawrence
Most every week during one of our alignment training classes, we’ll get a question about aligning cooling towers. Most facilities we visit have them and a lot of folks aren’t too sure how to approach the job. We’ll tackle the first step in this episode and explain a bit about roughing in the motor to the gearbox.
As with all successful alignments, planning the job beforehand can save a lot of headaches–especially when it comes to cooling towers. The long span of the spacer shaft, the physical separation of the motor and gearbox, limited access or tip-toeing on concrete beams present a challenge. Here are a few pointers to get you started:
Rough-In – Roughing-in should be done to minimize the possibility of base-bound or bolt-bound conditions, and should be done while the fan is in construction or commissioning stages, or after a gearbox or motor replacement. The roughing-in process for a cooling tower fan with a spacer shaft is different than a standard motor-pump configuration since there may be several feet between the motor and the gearbox–and a 15 foot long straightedge just won’t work!
- A NOTE FROM THE REAL WORLD: It is not difficult to rough in a spacer shaft with a dial or vernier caliper, by measuring the gap difference top to bottom, and side to side, for each coupling. Quite honestly, you can perform an adequate alignment using this process if your math skills are good, and if you take great care to keep up with your numbers. But it is much easier, much faster and requires much less ladder climbing to go back and forth between the couplings, to rough it in using the laser alignment tool.
It is also very common for a cooling tower drive to be bolt-bound, base-bound, or both. Don’t be afraid to move or shim the gearbox to eliminate a bolt-bound or base-bound condition. If you must move the gearbox horizontally to correct a bolt-bound situation on the motor be mindful of blade tip clearance around the plenum. Also, if you experience a base-bound condition, where the motor is sitting too high, you can shim the gearbox slightly just under the inboard end of the gearbox.
A cooling tower fan using a spacer shaft can be roughed in with a laser alignment tool, using the following process:
- With your lasers in the 12:00 position, center both lasers onto the targets by adjusting the laser transmitters/receivers.
- Rotate the shafts to the 6:00 position.
- Note the location of the beam on the moveable detector. Adjust the position of the beam striking the moveable detector by moving the entire motor up or down half the distance of the change.
- Return to the 12:00 position, and re-center both detectors.
- Rotate to the 6:00 position again.
- Note the location of the beam on the stationary detector. Adjust the outboard motor feet to move this beam halfway back to its original position.
- Repeat the process at the 3:00 and 9:00 positions, to rough in alignment horizontally.
If your lasers have sufficient target size to measure at least 90° of rotation, skip all of this, and let the laser do the roughing in and the alignment in one step!
Once the roughing in process is completed, or if you have sufficient range to take an initial measurement, continue with your alignment as you normally do.
There will be more on cooling tower alignments (which can be applied to all spacer shaft alignments) but if you have a question in the meantime, leave us a comment.