When working with less experienced engineers, better known as young engineers, I try to pass on some tricks of the trade, most recently when a colleague was reviewing some shop drawing submittals for a project we were working on.
After discussing what I look for, relative to design specification and equipment schedule compliance, I began to address my “looking for elephants” items, better known as potential problems, beginning with AHUs. Here are the results:
- Design selection (cfm and static pressure) too far to the right on the fan curve will probably mean the fan is operating at an excessive fan rpm, resulting in additional fan hp. My suggestion is to consider going to the next size larger fan.
- There may be an excessive amount of noise associated with this point on the fan curve from both the fan rpm speed and the fan outlet velocity. This problem needs to be addressed before leaving the design phase, or even worse would be addressing it after the unit is up and running. The cooling coil selection that will come with this AHU may have both excessive high coil velocity to the point where moisture carryover will occur during peak A/C periods and with higher than needed air static pressure drop due to the coil velocity.
- Design selection (cfm and static pressure) to far to the left on the fan curve will probably mean the fan is outside the manufacturer’s recommended selection range (aka, unstable point on the fan). If the manufacturer’s engineer doesn’t point this out to the design engineer, the fan could pulsate while operating, resulting in long-term wear on the fan. You don’t want to find that out at equipment startup.
- Design engineers usually select AHU fan based on dirty filters, but systems are started and balanced using clean filters. If the system doesn’t have a VSD to slow the fan speed down, the unit may move more air than needed, consume more motor hp than needed, possibly have high enough coil velocity to have moisture carry-over, and may generate more airborne noise than planned. The alternative is to replace the new fan sheave with a more appropriate sheave but who will pay to buy it and install it?
- When selecting the fan based on worst-case scenario (dirty filters), the designer needs to take into account that the unit will be started and will operate initially with clean filters. Without the correct automatic controls, the fan may move down its fan curve and operate beyond the motor selection, resulting in fan motor overload. Eventually, the fan will shut down due to the overload condition.
- Fan rotation, whether within a fan casing enclosure or a utility set, can be an important item that gets overlooked. For example, all too often, I will see a fan selection such as top-up blast specified as counter-clockwise when the drawing shows the fan with a clockwise rotation. The result will be a fan discharging up and taking a hard right turn when the air really would like to go left. Under these conditions, the arrangement could be adding a half-inch of static pressure to the system that the designer didn’t count on, and the building owner will pay for in additional motor energy consumption for the life of the fan.
- Staying with the wrong rotation dilemma, frequently the mistake doesn’t get picked up until the fan has reached the site. When this happens, I have seen the fan get turned in the correct direction for efficient fan discharge flow, but the motor location ends up on the opposite side of the fan in an unintended and probably inaccessible spot blocked off by ductwork and pipes.
Pay attention to details when you are selecting, scheduling, specifying, and/or laying out an AHU or a utility fan set. Taking that last-chance look at the AHU or utility set shop drawing submittal is very important to achieving the efficiency of the fan performance over the life of the system. When the designer invests the time to lay out the unit correctly but overlooks pertinent details in the specifying of the fan (e.g., clockwise vs. counter-clockwise fan rotation), as well as making sure the correct motor location is specified for ease of servicing, then problems on paper become problems in the field. Problems in the field can become problems for O&M, resulting quite often in wasted energy over the life of the fan.
For one more design-to-construction oversight check out the “What’s Wrong With This Picture” blog page and see how one fan was installed and think about how the maintenance technician will service this unit in the years to come. ES