Chapter 2: Fan Casing

2.1. Theory and Application

2.1.1. Construction

2.1.2. Shipping and Assembly

2.1.3. Location

2.2. Commissioning the Fan Casing

2.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

 

Figures

Figure 2.1: Casing Joint Damage

Figure 2.2: Typical Air Handling Unit Field Assembly

Figure 2.3: Extruded Caulk Points to Missing Bolts

Figure 2.4: Field Damage

 

2.1.  Theory and Application

2.1.1. Construction

On a project with large packaged rooftop units, a commissioning provider aggravated the projectís experienced lead electrician by insisting that they open the 480 volt electrical compartment to verify the security of the wiring connections prior to energizing the unit for start-up.  When they opened the compartment, gallons of water spilled out because the gasket on the top part of the service panel allowed rainwater from a week of thunderstorms to enter and accumulate in the compartment.  This situation convinced the electrician that checklists might not be such a bad thing after all. 

Fan casing requirements for an air handling system can vary from none for a simple ventilation system supplied by a thru-the-wall prop fan and exhausted by a small utility fan to extremely sophisticated for a custom roof mounted air handling package that includes service corridors and all of the related heating boilers, water chillers pumps, piping and electrical equipment necessary for operation. 

Casing configurations and quality can vary over a wide range.  At the low cost, less sophisticated end of the spectrum, the casings provided with packaged equipment are generally weather resistant, easily shipped, and easily installed.  At the upper end of the spectrum, fully customized air handling packages are available from a variety of manufacturers.  Typical configurations are available for standard equipment rooms or a custom approach can be fabricated to enhance maintenance and durability with features like insulated double wall construction, walk-in man doors, internal lighting and convenience outlets, and all aluminum construction.  Top of the line custom packaged units may also include factory installed controls, wiring, pumps, boilers, chillers and other processes, which only need to connect to a source of electricity and fuel to function.

The modular packaged equipment offered by many of the major manufacturers provides a good compromise between the two ends of the spectrum.  Designers can pick and choose from a wide array of standard building blocks to assemble a unit that meets their specific project needs.  Many features like double wall construction, factory wired controls, variable speed drives, and internal lighting and convenience outlets are available as standard options in these product lines.

Regardless of the quality level, one of the important features that should be included in a double wall construction system is providing a thermal break in the channel and framing system that used to assemble the sections.  Without this break, there can be significant condensation problems for units installed in unconditioned areas.  The thermal losses associated with this are not nearly as significant as the potential for water damage and moisture related IAQ problems that can be caused by the condensation.  This is a good item to include in a design phase commissioning review of the project documents.

2.1.2. Shipping and Assembly

For most product lines, shipping limitations will typically result in larger air handling units being shipped in several pieces.  In most instances, the units are assembled at the factory to ensure that everything lines up, then disassembled and shipped.  While performing construction observation, it is often wise for the commissioning team to spot-check the field assembly process during the following steps:

       Move the sections into place This is an obvious first step, but is not without its potential pitfalls.  Many of the sections in a large air handling unit can look similar, especially to the teams rigging them into place whose expertise lies in moving heavy machinery rather than in air handling system design.  Mislabeling at the factory or confusion in the field can lead to sections being assembled in the wrong order, sections from different units being assembled together, and units being installed at the wrong location.  If any of these problems occur, it will only get worse as final connections are made and other equipment is moved into place.  Catching this type of problem early can save everyone a lot of trouble.

Figure 2.1: Casing Joint Damage

The joint between these two sections was damaged during assembly.  The threaded rod between the lifting lugs was used to pull the joint back together again.

       Prepare joints for assembly Usually this step consists of applying a gasket or caulk to the mating surfaces of the joint.  After final alignment, the mating surfaces are pulled together.  If a heavy section is being pulled up to mate with a light section, there may be a tendency for the light section to move, which can deform the connection between the light section and the module it is connected to at the other end (see Figure 2.1).  This problem leads to misalignment with openings in the building structure arranged to allow utility and duct system connections to the unit.

       Install the mechanical connectors After the modules are in their final locations, they can be physically connected to each other.  Usually this involves installing bolts through flanges around the perimeter of the unit.  The exact requirements will vary with the manufacturer and the configuration of the unit, but there will usually be a bolted connection on at least one if not all sides. 

       Install and joint covers Most manufacturers provide sheet metal covers that are caulked and then fastened into place over the field joints.  These covers can be critical to maintaining the joints integrity and preventing moisture penetration into the insulation for units that are located outdoors or in unconditioned environments.  The roof joint is particularly important in this regard.  Inverted U caps are often employed over standing flanges for this location.  Some high-end manufacturers install a continuous rubber membrane type roof after the sections are assembled on units located outdoors.  In this particular situation, the integrity of the roof joints can be important in terms of preventing air leakage from blowing the roofing material off of the unit.  Thus, it may be desirable to perform any casing leakage tests prior to installing the membrane roof, so leaks can be detected and repaired before they have a chance to cause a problem with the roofing system.

Figure 2.2: Typical Air Handling Unit Field Assembly

A and B - The two sections that are to be joined.

C and D - The flanged joint on the bottom of the unit before and after bolt-up.  Note the evidence of caulk after assembly (inside the red circle).

E - Side and top joints at the flange location.  This unit only had a bolted flange on the bottom.  The sides and top were caulked but had no other fasteners other than the external joint covers and inverted U channel on the roof.  Note the diagonal corner brace at the top of the unit (inside red circle).

F - The completed joint with joint covers in place.

Ideally, it is best to witness this process as it occurs, but in most cases, assembly problems can be detected after the process.   For example:

       Empty holes Empty bolt holes are evidence of missing bolts.  While it is possible that there are holes that were not intended for mechanical fasteners at a field joint, it is unlikely.

Figure 2.3: Extruded Caulk Points to Missing Bolts

       Extruded caulk Extruded caulk can show that efforts have been made to properly seal (Figure 2.2) as well as point to missing fasteners (Figure 2.3)

       Nuts and bolts The type of fastener installed in a flange can also be an indicator of a problem.  Most (but not necessarily all) flanged joints are designed to be used with bolts.  A zip screw installed in a hole in a flange that was intended for a bolt can be a recipe for disaster waiting to happen.

Figure 2.4: Field Damage

If the units are going to be sitting on site or in a storage yard for a while before being placed into operation, it is important to periodically inspect them.  Items to check for include:

       General condition Look for evidence of damage while in storage like broken windows.  Catching problems such as the broken windows shown in Figure 2.4 as soon as they occur will speed resolution, prevent them from becoming an obstruction to start-up, and prevent other problems such as water accumulation.

       Moisture accumulation and condensation The temperature and humidity changes that can occur in non-operating units stored outdoors and other uncontrolled environments during construction can cause condensation internally, even if the units are properly covered and protected.   This condensation can cause corrosion on the components and could lead to mold and mildew growth.  If this occurs, it may actually be necessary for the contractor to temporarily remove covers and tarps that are protecting the unit to allow it to air out.

       Bearing care Bearings that sit in one position without rotating for a long period of time coupled with a moist environment can develop a form of corrosion called false brinnelling as a result of metal to metal contact between the race and roller that occurs as the lubricant is gradually squeezed from between the two surfaces by the weight of the inactive shaft and its related assembly.  Thus, it is often a good idea to manually rotate inactive shafts on occasion to spread out the lubricant and maintain a film on all bearing surfaces.

2.1.3. Location

In addition to being a large energy consumer, the fan and its related air handling unit casing represent a significant assembly of raw materials.  In contrast to some HVAC machinery (a chiller for instance), air handling units are relatively simple machines with few moving parts or complex assemblies.  Equipment location is an important consideration in light of the longevity of the air handler.  Units located outdoors will deteriorate at a faster rate than those located indoors. 

Equipment longevity can also be ensured by configuring the air handling unit assembly in a manner that promotes ongoing maintenance and provides enough flexibility to allow the system to meet the changing needs of the building over time.  A long-lived unit optimizes the use of the resources, which is more sustainable than replacing the entire assembly.  Commissioning providers are in an ideal position to help ensure that the system is configured and installed in a manner that will provide for good O&M and adaptability.  Many of these issues are best addressed during design phase commissioning and supplemented by follow-up during field inspections over the course of the construction project.  

2.2. Commissioning the Fan Casing

The following tables outline the benefits and background information associated with testing fan casings.  These tests can be used in a retro-commissioning process to define and correct existing operational issues.   The tables are linked to related information throughout the Guide.  Refer to Functional Testing Basics for guidance related to all functional testing activities, regardless of the component or system being tested.

2.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Casing leakage and deflection test results should meet or exceed the project specification requirements.

Ensure that operation and service considerations are taken into account during design and construction of the air handling unit. This will help ensure the persistence of the efficient operation of the unit and prevent IAQ problems.

Verify casing integrity to ensure that leakage and thermal losses are minimized.

Key Preparations and Cautions

Construction Prefunctional Checklists and Start-up

Tests to verify design parameters for the enclosure can be performed after the assembly of the air handling unit but prior to its start-up

Test Conditions, Considerations, and Cautions

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

Casing leakage and structural integrity tests will subject the assembly to design static pressure values. Caution should be used when applying and removing these pressures to prevent damage due to sudden changes or assembly errors.

Caution should be exercised when opening and closing access doors on pressurized casings to prevent injury via a door slamming on the person using it. Caution should also be exercised to ensure that test personnel do not become trapped inside the unit by the pressure differentials

Instrumentation Required

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

A duct leakage testing machine to test the leakage rate from the air handling unit casing. This is often available from the sheet metal contractor, but may require certification if it has not be certified recently. It is also possible to field erect a test rig using a small variable speed utility fan equipped with appropriate duct connections that allow it to be installed on one of the air handling unit's access doors and provide for accurate flow measurement.

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

The structural integrity test involves pressurizing the air handling unit casing to specific levels and then measuring deflections. The duct leakage test rig can provide pressurization. Vernier calipers or other precision measuring equipment will be required to measure the casing deflections for comparison to design values.

Time Required to Test

The time required to test can be several hours to a day for a team to verify casing leakage and structural integrity.