Chapter 10: Fans and Drives

10.1. Theory and Applications. 1

10.2. Commissioning Fans and Drives. 2

10.2.1. Functional Testing Field Tips. 3

Key Commissioning Test Requirements. 3

Key Preparations and Cautions. 3

Time Required to Test4

10.2.2. Design Issues Overview.. 5

10.3. Typical Problems. 8

10.3.1. Static pressure requirements in excess of design. 8

10.3.2. Improper belt drive system adjustment8

10.4. Testing Guidance and Sample Test Forms. 9

10.5. Supplemental Information. 10

 

10.1. Theory and Applications

The fan is the heart of the air handling system since it is one of the most significant energy users in a building. Commissioning and re-commissioning fans and drives is a key factor for ensuring that a buildingís efficiency goals are met over the life of the building.

There are both indirect and direct components to a fanís energy consumption. The indirect component relates to the system the fan serves. The fan must impart enough energy to the air stream to overcome the systemís resistance to flow. This energy consumption can be significantly altered by:

       Fan installation considerations like system effect

       Duct and fitting design and their related pressure drops

       Component pressure drops

       Duct system leakage

       Duct system thermal loss

These topics are discussed in Chapter 11: Distribution, and Chapter 13: Return, Relief and Exhaust System.

The direct fan energy component relates to how efficiently the fan can covert the energy going into its prime mover (usually electricity into a motor) into air flow and pressure in the fan system. This energy consumption is a function of the following items:

       Fan efficiency

       Motor efficiency

       Drive system efficiency and adjustment

The fan horsepower equation (Equation 10.1) is a function of several fundamental components: flow rate, static pressure, fan efficiency, and motor efficiency. Application of this equation to fan system analysis is discussed in detail in Appendix C: Calculations. Commissioning efforts should be targeted at these factors to ensure system efficiency, performance, and reliability.

Equation 10.1 Fan Horsepower

The fundamental technology associated with the fans, coils, casings and other major system components is well established. Most of the advances in technology that improve the performance of these components are related to the drive systems and control systems, not with the components themselves. Drive and control systems can be readily upgraded as technological improvements warrant. This easy upgrade is in direct contrast with a more complex machine like a chiller where technology changes are continually improving performance, and where this technology is generally an integral part of the machineís package.

If one examines a reasonably well-maintained 50-year-old airfoil centrifugal fan and a similar unit right off of the production line, one will probably see only modest performance differences. Over time, the shaft and/or bearings in the older fan may have required replacement, and the wheel periodic cleaning. However, it is likely that the fan is capable of moving air as efficiently as the newer fan. Through attention to proper maintenance and equipment location (indoors rather than outdoors), fans can be a lasting component of the air handling system.

10.2. Commissioning Fans and Drives

The following sections present benefits, practical tips, and design issues associated with commissioning an air handlerís fans and drives.

10.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements lists practical considerations for functional testing. Key Preparations and Cautions address potential problems that may occur during functional testing and ways to prevent them.

Key Commissioning Test Requirements

Fan energy constitutes a significant portion of a building's overall energy consumption. Even minor improvements in efficiency can have a major impact on the energy consumption pattern associated with a building. A well executed commissioning plan for the fans and their associated drive systems ensures that the systems are set up for peak efficiency and that this efficiency will persist. The fan and drive control should reliably integrate with the overall system control strategy in a manner that provides the intended function and level of performance.

1 Verify the fan size and capacity. Capacity tests results should be evaluated in the light of the accuracy of in instrumentation and the actual conditions at the time of the test.

2 Backdraft dampers need to be tested for proper operation. Non-motorized dampers must open and close freely without binding. Motorized dampers must be connected to the DDC control system and verified that they are commanded open prior to fan operation.

3 Verify that network failures do not result in unsafe operating modes. The recovery from the failure should safely return the drive to the network.

4 Verify that drive settings and adjustments provide for safe and reliable system operation at peak efficiency levels in all operating modes.

Key Preparations and Cautions

Cautions

1 Applicable cautions as outlined in Functional Testing Basics should be observed.

2 Safety and interlock testing, verification of some of the drive settings, and loop tuning efforts will place the system at risk. Appropriate precautions and procedures should be in place to protect the personnel and machinery involved in the test process, including plans for quickly aborting the test.

3 Any belt drives have been adjusted and aligned.

5 All safeties, interlocks, and alarms are programmed (or hard-wired, if applicable) and function correctly, regardless of VFD operating position (i.e. hand, auto, by-pass).

6 If necessary, the motor shaft is grounded.

7 Distribution system pressure drops do not exceed design expectations.  This is typically performed while conducting construction observation. If changes increase distribution system pressure drop, ensure all equipment still receives design flow rate.

8 Verify all VFD operating parameters are correct for the application, including acceleration and deceleration times and minimum speed setting.

Test Conditions

1 Tests that are targeted at verifying design parameters and settings for the fan and its enclosure can generally be performed after the assembly of the air handling unit but prior to its start-up.

2 Other tests targeted at the interlocks and fundamental control functions, loop testing and tuning, and capacity testing will require that the air handling system be operational and moving the design volume of air, but not necessarily fully under control. Safety systems should be operational to protect the machinery and occupants in the event of a problem during the test sequence.

3 Testing the integrated performance of the fan and drive with the rest of the system will require that the individual components of the system be fully tested and ready for integrated testing. In many cases, testing integrated performance will also require that at least the portion of the building served by the system be substantially complete and under load.

4 For variable flow systems, system testing may often involve forcing an individual system into a full flow configuration. Usually this requires delivering flow in excess of the current requirements in many of the zones. Manual adjustments to discharge temperatures and local reheat valves can mitigate the impact of this on occupants, but some comfort problems may be created, especially if adjacent systems are operating under normal conditions. In situations where control of building pressure relationships is critical, forcing the system to run at full flow when it is not required by the actual load conditions may have adverse effects on the desired pressure relationship. These issues should be taken into consideration when developing the system test plan.

5 System should operate in by-pass mode without severely damaging the distribution system or equipment.  For example, system pressure may exceed the high static pressure limit switch (or worse rupture the ductwork or equipment) if the fan is operating at a constant speed in by-pass mode but the zone VAV boxes are restricting air flow to meet space temperature setpoint.  The case may be that the fan can only operate safely in by-pass mode, if all zone VAV box dampers are commanded 100% open to prevent nuisance trips of the high static pressure switch or damage to the systems.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will include the following in addition to the standard tool kit listed in Chapter 2: Functional Testing Basics:

1 Inclined manometers, Magnahelics, Shortridge Air Data Multimeters, and other instruments capable of measuring and documenting low air static and velocity pressures.

2 Most current technology inverter-type variable speed drives can be programmed to indicate their current power consumption. For other VFD technologies and for non-drive equipped systems, a data logger capable of continuously monitoring three phase power and other related parameters is useful for optimizing the system's efficiency and documenting the operating performance in various modes.

Time Required to Test

1 The time required to test can vary from less than an hour for one person for simple interlock verifications up to a day or more for a team to verify the fan's integrated performance with the air handling system.

2 The time associated with the integrated system testing will not be purely focused on the fan and will yield benefits in relation to other system components and functions. Thus, when budgeting for this sort of testing, it is reasonable to assume that a number of components and their interactions will be tested simultaneously.

3 Capacity testing and integrated performance testing will involve coordinating the activities of multiple parties. This coordination should be taken into consideration when developing the projects commissioning budget and schedule.

 

10.2.2. Design Issues Overview

The Design Issues Overview presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand, even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing.

Does the unit have good access for control installation, maintenance, and component replacement?

Access to the fan and its related components is critical for ensuring the persistence of energy efficiency and other commissioning related benefits.

1   Piping should be arranged to ensure that access panels are not blocked, service routes remain open, and components such as coils and fan shafts can be removed and replaced without shutting down adjacent systems or central plant equipment.

2   Fan scrolls should be provided with access doors to allow the wheel to be inspected and cleaned.

3   Coils should be provided with space between then and access to that space to facility inspection and cleaning and allow for the installation of control elements in their proper location. For example, space is required between a preheat coil and the next coil downstream to allow the freezestat to be installed downstream of the preheat coil (which by design will see subfreezing entering air temperatures and should be capable of handling them safely).

Have variable speed drive installation and operation requirements been taken into account?

1   Most VFD manufactures have some specific requirements regarding the length, routing, and general configuration of the power circuit from the drive to the motor. Failure to pay attention to the requirements can cause operational problems in the electrical system and in severe cases, cause failures in switchgear, drives, and transfer switches.

2   Many VFDs can be damaged if they start against a reverse spinning motor. This condition is likely to occur in parallel fan systems, even if they are equipped with backdraft dampers. No damper is 100% leak proof, and it does not take much reverse flow to set a fan wheel in motion. Most drives also have a feature to handle reverse flow, usually called DC injection braking. The process pulses the motor with a DC signal before starting and accelerating it. The DC signal brakes the rotating armature. Usually there are adjustments that need to be made to tailor this feature to the load served in addition to activating it. Verifying this feature is properly set and functioning should be part of the commissioning process both during the pre-start checks as well as the functional tests.

3   Many drives are supplied with bypass contactors that allow the motor to run at full speed if the drive fails. In some cases, the system could be damaged by full speed fan operation when the loads were configured for minimum flow conditions.

4   The drive should be configured and wired to ensure that all safety interlocks are effective in all possible selector switch configurations (local, auto, hand, inverter, bypass, etc.). Some drives are arranged to allow the safety interlocks to be effective when the drive is operating but not effective if the drive is bypass. Some drives can also be configured so that if they are placed in the local mode, any external interlock (external to the drive circuit board) will be ignored. This feature may be desirable in process applications, but it is highly undesirable in most HVAC applications. Verifying that the drive is properly set and functioning should be part of the commissioning process both during the pre-start checks as well as the functional tests.

Are the VFDs and the motors compatible?

Motors that are not rated for VFD applications may have a reduced life if used with a VFD. In retrofits, it is desirable to evaluate the motorís capabilities relative to the drive. Even it budget constraints prevent a motor replacement when the drive is installed, the potential for a future problem and early failure can be anticipated. In new installations, the drives and motors should be coordinated to be compatible with each other.

Does the VFD shaft need to be grounded?

The variable voltages, magnetic fields and harmonics associated with VFD operation can induce currents in the motor shaft that have no path to ground other than through the bearings for most conventional motors. Evidence suggests that these eddy currents can lead to premature bearing failures, perhaps in a matter of years on some motors. Shaft grounding kits installed on the motor provide a direct path from the shaft to ground via a brush system.

Is the drive arrangement suitable for the application?

Given the wide array of drive options available, it is important to tailor the selection to the application.

1 If direct drives are applied, then fan speed adjustment for balancing purposes will have to rely on less efficient approaches like discharge dampers, or will require that a variable speed drive be included as a part of the package.

2 A variable speed drive on a constant volume fan may represent false economy. While it does minimize balancing efforts and eliminate the need for a final sheave change or adjustment to set the fan speed, the drive results in a loss in fan system efficiency that increases in magnitude as the speed is reduced (see Chapter 10: Fans and Drives Supplemental Information). The drive also introduces operating complexity, first cost, potential electrical system harmonic problems, and multiple failure mode possibilities into the system. These issues coupled with the efficiency reduction will probably outweigh any modest savings in balancing costs achieved.

3 Variable pitch sheaves provide flexibility and a good intermediate stepping stone between start-up and final balanced speed as a system is brought on line. But some of their disadvantages may make the installation of a fixed pitch sheave as the final step in the balancing effort a desirable feature to include in the project.

4 During design review, verify that the fan and drive capacity is properly sized so that the VFD will operate near 100% speed at full load (do not use the VFD as a throttling device).

Could the fan motor run in the wrong direction?

For most axial fans, if the impeller were to run in the opposite direction, it would move air in the opposite direction. With centrifugal fans, running the impeller backwards will still provide flow in the correct direction, but the performance will be degraded significantly.

Reverse flow or back-draft through most fan wheels will cause them to spin in the reverse direction. Forward curved fan wheels will spin in the wrong direction if air is blown through them in the right direction but they are not energized. For most single phase motors, if the motor is spinning in the wrong direction when power is applied, the fan will simply run in the wrong direction. The rotational direction of most three phase motors used for HVAC applications is tied to the phase rotation established by the way the windings are connected to the distribution system. Thus, if the motor is spinning backwards when voltage is applied, it will reverse and run in the proper direction. Problems can occur with variable speed drives when they attempt to start against a reverse rotating motor.

Systems with operating conditions that could cause backflow should be designed and installed to safely and reliably deal with any problems. Both normal and failure modes need to be considered. Common examples of situations where backflow potential exists include:

1   Systems with parallel fans or air handling units. Donít forget that parallel fan terminal units have fans that are essentially in parallel with the supply fan.

2   Systems with series fans: the supply and exhaust fans associated with a 100% outdoor air systems and the fans in series powered fan terminal boxes relative to the supply fan.

Does the air handler specification include desirable options?

Most fans and air handling units are available with an array of options, some of which are desirable in most installations and others of which are only required for special installations. Examples include:

1   Access doors in casings and fan scrolls.

2   Lubrication lines extended to be accessible from the exterior of the unit.

3   Baseline vibration characteristics measured at the factory.

4   Premium efficiency motors.

5   Special vibration isolation provisions.

6   Scroll drains (essential for exhaust fans located outdoors and discharging in the up-blast configuration)

7   Factory installed back draft dampers.

8   Non- sparking or explosion proof construction for hazardous locations.

9   Special coatings for handling abrasive or corrosive fluid streams.

10.3. Typical Problems

The following problems are frequently encountered with fans and drives.

10.3.1. Static pressure requirements in excess of design

A typical problem found during commissioning or retro-commissioning is high static pressure in the fan system. In creating excess static pressure that is not required to operate the system, a fan wastes significant amounts of energy. This problem arises because fan selections often fall into a range where there is a difference between the design brake-horsepower (bhp) requirement and the actual motor horsepower installed due to the standard horsepower ratings available in motor product lines. The difference between available sizes can become quite significant for larger fans. For example, a fan with an 82 bhp motor requirement would probably come with a 100 hp motor. If the fan was unable to deliver design flow against the installed system static requirement, then there would be a lot of margin for speeding the fan up to achieve the design requirement without overloading the motor (assuming the operating point did not end up in a different fan class requirement). This safety net may be desirable, as the excess motor capacity allows problems to be solved in the field. But, the added energy consumed by the fan beyond that intended by the design will become an energy burden that will persist for the life of the system.

Extra diligence during design and construction can prevent conditions that add unanticipated static pressure to the system, thus averting the need to run the fan at an operating point in excess of design. If the balancing team discovers that they have excess system static pressure, there are ways to lower static pressure that will allow the system to function at or near its intended design point rather than adding on ongoing energy burden to the project by simply throwing energy at the problem. An example of such a situation is contained in Example 2 in Appendix C : Calculations.

10.3.2. Improper belt drive system adjustment

While simple in concept, there are some critical parameters associated with the installation and adjustment of this belt drive systems that are often ignored, resulting in belt failures, poor performance, noise, reduced equipment life and energy waste.

Alignment of the drive and motor sheaves is a critical step in the belt installation process. Without proper alignment, belts will run less efficiently, wear out more quickly, and, in extreme cases, be thrown off the drive sheaves.

Over-tensioning the belts can cause problems with bearings and shafts due to the excessive loads imposed. In addition, new belts will stretch during the first 8 to 24 hours of operation; belts that have been properly set initially will require re-tensioning after they have run. This contingency is often overlooked to the detriment of the drive system efficiency.

Multiple belt drives will function best if factory matched belt sets are installed. This ensures that the drive loads are equally distributed between all of the belts, equalizing wear and life.

The T.B. Woods Company offers a very good guide to proper belt drive selection and adjustment on their web site.

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10.4. Testing Guidance and Sample Test Forms

Click the button below to access all publicly-available prefunctional checklists, functional test procedures, and test guidance documents referenced in the Testing Guidance and Sample Test Forms table of the Air Handler system module.

AHU Testing Guidance and Sample Test Forms

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10.5. Supplemental Information

Supplemental information for fans and drives has been developed to provide necessary background information for functional testing.

Bevel: Fans and Drives Supplemental Information