Gravity Vents

  • gravity vents GI GR
  • gravity vents PR
  • gravity vents TR
  • gravity vents TRE
  • gravity vents LSUG
  • Standard Construction Features
  •  Certifications Logo

Gravity vents GI GR

Gravity vents RR

Gravity vents TR

Gravity vents TRE

Gravity vents LSUG

Standard Construction Features

Standard Construction Features

AMCA-Sound & Air Cert. logo

ISO 9001, US-Green logos -by model

Natural cooling & contaminant dilution

ILLUSTARION of Natural cooling & contaminant

Gravity ventilation (also known as natural ventilation) requires no powered ventilators to achieve air movement. It is most useful in buildings with open floor plans requiring no air conditioning, and where the primary motive for ventilation is heat removal or contaminant dilution. Gravity ventilation is achieved by strategically placing openings in the building to take advantage of prevailing winds and thermal buoyancy or stack effect.

Wind pressure

Wind moving past the building creates areas of high and low pressure. The windward side of the building is an area of high pressure. The leeward side & roof are areas of low pressure. Placing openings in both high and low pressure areas of the building causes air to move through the building.

Thermal buoyancy (stack effect)

Heat generated within buildings rises up. In buildings where the structure is of significant height, this natural tendency of warm air to rise (known as thermal buoyancy or stack effect) can generate air movement in the building. By placing openings in both the highest and lowest levels of the building, hot air is allowed to rise out of the upper part of the building and pull cooler air into the lower level of the building.

Opening placement

In order to take advantage of both wind pressure and stack effect, gravity ventilators are generally used in conjunction with open doors and windows to ventilate the structure. Gravity vents are placed on the roof, being the highest part of the structure, to provide for exhaust of building air. Windows and doors at lower elevations provide the intake portals of the gravity ventilation system.

Use with mechanical systems

Gravity ventilators may be used with mechanical ventilation systems, where power ventilators force air in or out of the building through the gravity vents. The power ventilator may be directly connected to the gravity ventilator via ductwork or the building may constitute a plenum, the air induced to move through the gravity ventilator merely by the pressure differential created between the interior of the building and the ambient atmosphere.


ILLUSTARION of Natural cooling & contaminant

Ducted relief

This application is the most simple of all gravity ventilator applications. The ventilators are selected based on the air volume and acceptable pressure drop along with any physical limitations which may affect the dimensions of the product.


ILLUSTARION of Natural cooling & contaminant

Ducted intake

Ducted intake applications are similar to ducted relief with the additional consideration of intake velocity. Moisture carryover begins to occur at approximately 500 FPM intake velocity and gravity ventilator selections should, therefore, be kept below that threshold unless moisture containment measures are added to the system.


ILLUSTARION of Natural cooling & contaminant

Non-ducted relief

Non-ducted relief gravity ventilator applications are very similar to ducted relief applications with the exception that the pressure drop of the ventilator should be kept to a minimum. The building will be pressurized to approximately the selected pressure drop of the gravity ventilator. Selecting the ventilator for pressure drops in excess of 1/8” wg will result in problems with exit doors standing open and unwanted infiltration into any adjacent conditioned areas.


ILLUSTARION of Natural cooling & contaminant

Non-ducted intake

Although the 500 FPM intake velocity will usually limit the intake static pressure, the designer should double-check gravity ventilator pressure drop to ensure that it does not exceed 1/8” w.g. If the negative pressure is excessive, it can result in difficulty opening exit doors and dangerous slamming of already open doors.

 

Related Documents

See the Downloads tab for specifications, submittals and FanDrafter files

  • GI / GR

  • PR

  • TR

  • TRE

  • LSUG

  • Selecting

  • Downloads

GI

All sizes are available with express service. Read more.

GI / GR

Gravity Vent

  • High efficiency gravity roof ventilators designed using the latest Computational Fluid Dynamic (CFD) and Finite Element Analysis (FEA) software packages
  • Bases are manufactured with a rounded flange around the throat that decreases pressure losses due to turbulence. (See CFD Models Below)
  • Available with throats in widths from 12” to 72” and unlimited length starting at 12”.
  • Units come standard with integral lifting lugs.
  • Tie down points are standard to allow for additional restraints to be added in the field as required.
  • Rain gutters are standard to help eliminate rain infiltration.
  • Roll formed hood panels provide excellent strength characteristics as well as creating a unit that is architecturally pleasing.
  • Units are designed to exceed 30 lbs./ft2 snowload rating.
  • Hinged hoods are standard on GR units with less than a 73” throat length and GI units with less than a 61” throat length.

Designed with the latest in computer aided engineering software.

The new COOK GI and GR Gravity Ventilators were designed using the latest Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) software. This allowed our engineers to optimize the geometry of the throat and hood to reduce pressure losses and increase efficiency, while maintaining structural integrity. Once the design was optimized, physical prototypes verified the performance and durability of the design. The result is the most efficient Gravity Ventilator in the industry.

CFD Simulation

The figures above depict the relative turbulence of each unit and illustrate the improved airflow achieved by the new COOK design.
As can be seen in the figures, the radius throat design significantly reduces turbulence as compared to the typical square throat design.

Description: Unit shall be a hooded high efficiency roof mounted gravity ventilator.

Construction: The unit shall be of bolted and welded construction utilizing corrosion resistant fasteners. The hood shall be constructed of minimum 18 gauge aluminum, bolted to a minimum 8 gauge aluminum support structure. A radius throat must be provided for optimum performance. Lifting lugs shall be provided to help prevent damage from improper lifting. A rain gutter shall be provided to prevent rain infiltration. The base shall have continuously welded curb cap corners for maximum leak protection. Birdscreen constructed of 1/2” galvanized mesh shall be mounted in the hood. Unit shall bear an engraved aluminum nameplate and shall be shipped in ISTA Certified Transit Tested Packaging.

Product: Units shall be model GI and GR as manufactured by LOREN COOK COMPANY of Springfield, Missouri.

Intake Hood Velocity Notes

Velocity to 500 FPM: Velocity below 500 FPM provides for a level of protection against the influx of airborne mist or debris.

Velocity 500 FPM to 1000 FPM: Within this velocity range there is a possibility that fine, airborne mist will be carried into the building through the vent. Care should be taken in the location of the unit if airborne mist is a problem.

Velocity Greater Than 1000 FPM: Velocity in this range is not recommended unless special means are taken to prevent the influx of airborne matter and large droplets of moisture into the interior of the building. Velocities shown are to be used as a guide only. Final application and conditions will be a governing factor.

PR

All sizes are available with express service. Read more.

PR

Spun Aluminum
Gravity Vent

  • Spun aluminum intake/relief ventilator designed for use in both gravity and mechanical ventilation systems.
  • Compact, durable and efficient.
  • Bases are manufactured with a fully developed inlet to provide the most efficient and weather resistant path for both supply and exhaust applications.
  • Available in 9 sizes from 8 to 48 inches.

Description: Unit shall be a spun aluminum, roof mounted gravity ventilator.

Construction: The unit shall be of bolted and welded construction utilizing corrosion resistant fasteners. The spun aluminum structural components shall be constructed of minimum 16 gauge marine alloy aluminum, bolted to a rigid aluminum support structure. The aluminum base shall have continuously welded curb cap corners for maximum leak protection. The spun aluminum baffle shall have a rolled bead for added strength. Birdscreen constructed of 1/2” mesh shall be mounted across the air opening. Unit shall bear an engraved aluminum nameplate. Nameplate shall indicate design CFM and static pressure. Unit shall be shipped in ISTA Certified Transit Tested Packaging.

Product: Unit shall be model PR as manufactured by LOREN COOK COMPANY of Springfield, Missouri.

TR

All sizes are available with express service. Read more.

TR

Spun Aluminum Tiered
Gravity Vent

  • Spun aluminum ventilator designed for use in both gravity and mechanical ventilation systems.
  • Provide an increased intake area by utilizing two spun aluminum tiers and therefore, provide lower intake velocities reducing the introduction of airborne particles or moisture into the building.
  • Bases are manufactured with a fully developed base venturi to provide the most efficient and weather resistant path.
  • Available in 9 sizes from 8 to 48 inches.

Description: Unit shall be a tiered spun aluminum, roof mounted gravity ventilator.

Construction: The unit shall be of bolted and welded construction utilizing corrosion resistant fasteners. The spun aluminum structural components shall be constructed of minimum 16 gauge marine alloy aluminum, bolted to a rigid aluminum support structure. The aluminum base shall have continuously welded curb cap corners for maximum leak protection. The spun aluminum baffle shall have a rolled bead for added strength. Birdscreen constructed of 1/2" mesh shall be mounted across the air opening. Unit shall bear an engraved aluminum nameplate. Nameplate shall indicate design CFM and static pressure. Unit shall be shipped in ISTA Certified Transit Tested Packaging.

Product: Unit shall be model TR as manufactured by LOREN COOK COMPANY of Springfield, Missouri.

PR

All sizes are available with express service. Read more.

TRE

Extruded Tiered
Gravity Vent

  • Tiered aluminum intake/relief ventilator designed to be one of the most flexible designs on the market.
  • Can be tailored specifically to meet most building exhaust and intake applications.
  • The tiers are constructed of high quality extruded aluminum and provide for clean lines that can meet the requirements for even the most demanding architectural applications.
  • Available with throat dimensions from 12” x 12” to 60” x 120”

Description: Unit shall be an extruded aluminum, roof mounted gravity ventilator.

Construction - The unit shall be manufactured of 0.081 gauge extruded aluminum tiers welded to a minimum 8 gauge aluminum support structure. The aluminum hood shall be constructed of minimum 0.063 aluminum and provided with a layer of anti-condensate coating. The aluminum base shall have continuously welded curb cap corners for maximum leak protection. Birdscreen constructed of 1/2" galvanized mesh shall be mounted across the relief opening. Unit shall bear an engraved aluminum nameplate. Nameplate shall indicate design CFM and static pressure. Unit shall be shipped in ISTA Certified Transit Tested Packaging.

Product: Unit shall be model TRE as manufactured by LOREN COOK COMPANY of Springfield, Missouri.

Intake Hood Velocity Notes

Velocity to 500 FPM: Velocity below 500 FPM provides for a level of protection against the influx of airborne mist or debris.

Velocity 500 FPM to 1000 FPM: Within this velocity range there is a possibility that fine, airborne mist will be carried into the building through the vent. Care should be taken in the location of the unit if airborne mist is a problem.

Velocity Greater Than 1000 FPM: Velocity in this range is not recommended unless special means are taken to prevent the influx of airborne matter and large droplets of moisture into the interior of the building. Velocities shown are to be used as a guide only. Final application and conditions will be a governing factor.

Propeller Hooded H Series Fans

All sizes are available with express service. Read more.

LSUG

Low Silhouette Upblast
Gravity Vents

  • LSUG is a low silhouette, gravity relief, upblast ventilator designed for use on vertical high velocity exhaust systems.
  • LSUG units include damper doors and a high volume rain gutter to prevent water infiltration.
  • LSUG units are available in 7 sizes from 24 to 60 inches.

Description: Unit shall be a low silhouette gravity upblast ventilator

Construction: The unit shall be of bolted and welded construction utilizing corrosion resistant fasteners. The fan housing and base shall be minimum 14 gauge and the baffle shall be a minimum 18 gauge steel. Unit shall have built-in butterfly dampers of aluminum or steel construction (aluminum on sizes 24-36, steel 42-60). Curb cap corners shall be continuously welded for maximum leak protection. The wind band shall be continuously seam welded with rolled flanges on each end for strength.

Coating: All steel fan components shall be Lorenized® with an electrostatically applied, baked polyester powder coating. Each component shall be subject to a five stage environmentally friendly wash system, followed by a minimum 2 mil thick baked powder finish. Paint must exceed 1,000 hour salt spray under ASTM B117 test method.

Product - Unit shall be model LSUG as manufactured by LOREN COOK COMPANY of Springfield, Missouri.

Selecting

Cook has 5 models and many sizes of gravity vents. Use the equations below to understand which is the best.

First calculate the temperature differential (TD)

The temperature differential (TD) is the indoor temperature (TI) minus theoutdoor temperature (To) in °F. Indoor temperature should be lower than the outdoor temperature
- = °F


Then, choose an application to find the airflow

choose one

Heat removal

  • Heat load (HL) from sunlight, lights, motors, people etc. measured in BTU per hour.
    =
  • Airflow (Q) measured in cubic feet per minute (CFM) = HL / 1.1 x TD
    = CFM

Dilution of contaminants

  • Building volume (BV) in cubic feet
    =
  • Air change frequency (AFC) in minute per change
    (Q) =
  • Airflow (Q) measured in cubic feet per minute (CFM) = BV / AFC
    = CFM


Location & number

Once the required air volume is known, the locations of gravity ventilators and intake openings need to be determined. It is wise to disperse gravity vents over the roof surface to provide even ventilation. If all intake openings are located on one side of the building, typically the gravity vents would be located on the opposite side of the building to provide a sweeping effect across the space.


Finally, determine the size of gravity ventilators

  • Required airflow (Q) measured in cubic feet per minute (CFM). See above.
  • Wind velocity (V) in miles per hour. Most wind velocity data is taken from 33 feet above the ground. Wind velocity decreases closer to the ground. With any disruption, design wind velocity may be less than meteorological data. Typically half of the average wind velocity is a good design value.
  • Height of gravity ventilator above neutral pressure level (H) in feet. The height above neutral pressure level is assumed to be the distance above the centerline elevation of the largest opening in the side wall - often an overhead door. If the side wall openings are evenly spaced along the height of the side wall, the height above neutral pressure level would be half of the building height.
  • Gravity ventilator throat velocity (TV) in feet per minute
    equation =
  • Total gravity ventilator throat area in square feet (A) = Q /TV =
  • The throat area for each gravity roof unit in square feet = A / N

Look for vent(s) with Throat area (answer will calculate here) square feet

GI / GR: Up to 72 square feet throat area
PR & TR: Up to 12.9 square feet throat area
TRE: Up to 50 square feet throat area
LSUG: Up to 20.1 square feet throat area


Aspect ratio

Many selections are possible which will have an equivalent throat area. When making a decision on throat size, keep in mind that selections with an aspect ratio of 3:1 or less will perform the best. Aspect ratio is the ratio of the throat length to the throat width.

  • Example: a GI or GR 30" x 42" has an aspect ratio of 1.4:1 – well below 3:1.
  • PR, TR, & LSUG models always have a 1:1 ratio.
  • TRE modles are always 2:1 or lss.

Example

  • 90°F Indoor Dry Bulb Temperature
  • 80°F Outdoor Dry Bulb Temperature
  • 156,000 BTU/Hr Heat Load
  • 7 Gravity Ventilators
  • 5 MPH Wind Velocity
  • 15 ft Roof Elevation & 10 ft Overhead Door (Primary Intake)
  • throat area = 8.34
  • Select (7) LOREN COOK Model GR 30 x 42 (8.75 ft2 throat area).