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   Performance Evaluation of the Systems    

GUIDELINES BEFORE YOU BEGIN:
The testing and balancing of a system is necessary to insure proper and efficient operation of that system as it was designed. In any building where effluent and hot air is removed, that mass of air must be replaced to maintain a constant pressure in the space. Any change in the pressure differential between inside and outside air will in some way affect the operation of a system, most commonly that affect is a negative one.

A test and balance, as well as the simple performance test in International Mechanical Code section 507.16.1 should be included in all jobs, as code inspectors are increasingly enforcing these requirements. Requirement in the 2003 IMC, which is currently effective in most parts of the U.S., are as follows:

  "507.16 Performance test. A performance test shall be conducted upon completion and before final approval of the installation of a ventilation system serving commercial cooking appliances. The test shall verify the rate of exhaust airflow required by Section 507.13, makeup airflow required by Section 508, and proper operation as specified in this chapter. The permit holder shall furnish the necessary test equipment and devices required to perform the tests.

507.16.1 Capture and containment test. The permit holder shall verify capture and containment performance of the exhaust system. This field test shall be conducted with all appliances under the hood at operating temperatures. Capture and containment shall be verified visually by observing smoke or steam produced by actual or simulated cooking, such as with smoke candles, smoke puffers, etc."

 

The simplest means of doing the performance test is using a T-T Puffer from www.evhill.com. Activate the puffer and use it to trace effluent around the entire perimeter of the hood, emitting smoke a few inches under the lower edges of the hood. If the smoke goes into the hood, it passes the test. If smoke goes out of the hood, adjustment is needed.


  1. A hood with multiple make-up air risers should be balanced according to the cooking load beneath it. For example, if a hood with multiple make-up air risers has a charbroiler in the center and several ovens on the ends, the risers should be evenly balanced. This will achieve the most efficient contaminant capture.
    1. Perforated supply plenums discharging air around the hood should be set to the designed discharge velocity.
  2. When fan pulleys are adjusted, belts should then be re-checked for correct tension and an amperage reading should be taken on the motor to make certain it is not overloaded.
  3. The prime objective of balancing is to insure that each hood will capture all the contaminants produced by the equipment it covers without causing undesirable conditions in the kitchen (i.e., excessive negative pressure, excessive quantities of hot or cold air in the kitchen, etc.).
  4. A performance evaluation of the system can be performed only if all the following items have been completed:
    1. All fans operational and rotations visually verified by observation of the arrows stamped on them.
    2. All filters in place.
    3. Equipment under the hood in place and operational.
    4. HVAC units in place and operational.
    5. Exhaust hood performance can be seriously deteriorated by high velocity supply or makeup air sources near hoods. If there are problems with a hood performance test, ensure there are no 4-way or slot diffusers near hood. 2' x2' 4-way supply diffusers near hoods should be replaced with 2' x 2' perforated return registers (used as supply diffusers); these include no vanes, louvers, or other internal components to increase velocity.
If problems occur, refer to the Troubleshooting Section of this manual.


EXTERNAL FACTORS WHICH MAY AFFECT YOUR HOOD PERFORMANCE:
  1. HVAC units are generally specified to supply 25% outside air (OA) to the room ventilation. If RTU's are not supplying the proper amount of OA to the building, negative pressure will exist.
  2. HVAC return grilles located close to a hood can cause performance problems. The return grille competes with the hood to capture the air in the room. For example, a return grille for a 10-ton HVAC unit can draw anywhere from 3000 to 4000 CFM. This is equivalent to the exhaust of a 10' to 13' canopy hood. As a result, a return air grille located within 6' of a hood can have a serious effect on that hood's capture ability.
  3. HVAC diffusers located near a hood can create flows in the room that detract from the hood's ability to capture. If the HVAC diffuser bounces air off the front of the hood or directs air along the hood and past the end, the air flow created can draw smoke and contaminants out of the hood.


USING A SHORTRIDGE INSTRUMENT:
The Shortridge is a sophisticated instrument that, with its built-in features, is basically a self contained test and balance kit. It has a "velocity grid" for filter face readings, it has a "velocity probe" for ductwork readings, it has a "differential pressure" function to check room pressure and static pressure, as well as a temperature probe so it can calculate accurate values based on varying temperatures (most equipment assumes standard temperature and pressure), and many other useful features for helping in a test and balance.

Hood Information:

To Calculate the CFM, the following information must first be acquired:

  1. Hood size and length
  2. Filter size and position
  3. PSP width and length

Measuring Hood Static and Room Pressure:

Static Pressure:

  1. Measure hood static pressure at exhaust collar using the Shortridge instrument.

Static Pressure = _______

Room Pressure:
For an exhaust hood to work properly, the kitchen should be at a slight negative pressure (caused by its air removal), and the building slightly positive. The dining room should be a slight positive to the outdoors and the kitchen (+.02" w.c.) or about 300 cfms positive. This will keep dust and bugs outside and doors will be easy to open. The kitchen should be a slight negative to the dining room to keep odors in the kitchen. The kitchen should be balanced to slightly negative to the outdoors (0" to -.02" w.c.). Generally, if there is not enough negative, or if there is positive pressure there will be smoke roll out from the hood that occurs due to the wind currents from people moving around in the area, while if there is too much negative, there becomes a pressure problem on the building(opening doors, drafts, hot water heaters, etc.)

  1. Measure room static pressure using the Shortridge instrument. Adjust the supply fan to set room to 0.02" negative.

Room Pressure = _____



CALCULATING MAKE-UP AIR CFM WITH A SHORTRIDGE INSTRUMENT AND AVAILABLE EXCEL SPREADSHEET.
  1. Use the Velgrid mode to measure the supply air velocity, making sure to take measurements every twelve inches along the length of the PSP.
  2. Locate the appropriate Excel spreadsheet for the PSP/Supply velocity and record all necessary data. This spreadsheet will calculate the total supply CFM for the hood.

CALCULATING MAKE-UP AIR CFM WITHOUT A SHORTRIDGE INSTRUMENT
  1. Compute the open area of the supply plenum of the hood. This area must be calculated at the same plane that velocity readings are taken. Area can be calculated using the following formula:
    Area (ft^2)= Length (ft) x Width (ft)
    If both the length and width are measured in inches, use the following formula:
    Area (ft^2)= Length (in) x Width (in) / 144
  2. Record velocity of air through supply openings from left to right on raw data sheet.
  3. Compute and record average velocity through supply openings.
  4. Compute and record CFM through supply openings. CFM = Free area x average velocity.
  5. Compute total CFM through all supply openings for each hood.


CALCULATING EXHAUST AIR CFM WITH A SHORTRIDGE INSTRUMENT AND AVAILABLE EXCEL SPREADSHEET.
  1. Use the Velgrid mode to measure the velocity at each filter.
  2. Locate the appropriate Excel spreadsheet for the exhaust velocity and record all necessary data. This spreadsheet will calculate the total exhaust CFM for the hood.

CALCULATING EXHAUST AIR CFM WITHOUT A SHORTRIDGE INSTRUMENT
  1. Record filter sizes of each hood on raw data sheet.
  2. Compute free area of filters.
    Example:
    16 x 16 = 14 x 14 = 1.36 ft2
    10 x 20 = 8 x 18 = 1.00 ft2
    12 x 16 = 10 x 14 = 0.97 ft2
  3. Record velocity of exhaust gases through filters starting top left to right (5 reading/filter).
  4. Find average velocity through each filter.
  5. Compute CFM through each filter. CFM = Free area x average velocity.
  6. Total exhaust CFM for each hood.
  7. Multiply total exhaust CFM x 0.78. (This is the K Factor necessary when using the EDRA velometer.)


CONCLUSION:
  1. Compare specified data to the data recorded. Adjust exhaust as necessary using adjustable pulley on fan. Adjust supply as necessary using dampers on supply risers and adjustable pulley on supply fan.
  2. After setting hoods to specified data, the room parameters should be checked.
  3. If room parameters are not acceptable yet, the hood can be modified to improve them without decreasing hood performance. This is an acceptable condition.
  4. Use a smoke bomb to verify that the hood captures adequately. This can be your final verification.