Industrial Air Make-Up Units for Indoor Air Control

NISCO Co. is a distributor and engineering representative for Direct-Fired Make-Up Air Systems for air temperature control at large industrial plants.

Direct Gas-Fired Heating Systems may be built in a number of configurations. They may be curb mounted as horizontal rooftop systems with down or horizontal discharge or in a vertical configuration for mounting on a pad outside of a building, with air ducted to the interior. They may also be located inside a building, suspended from hangers or mounted on support steel. Various manufacturers can supply any physical configuration required.

Two air flow configurations are available: 100% Fresh Air or Recirculating.

In the 100% Fresh Air arrangement, air is heated as it passes through the unit and supplied to the building space.

In the Recirculating arrangement, a damper is installed to allow a portion of building air to be supplied to the unit and returned to the building. This air is usually introduced downstream of the burner in a pull-through unit. Various percentage settings are available: 80% return / 20% fresh, 50/50, or variable.

It should be noted that demand for recirculating direct gas-fired systems has fallen in recent years. Indoor Air Quality concerns, more stringent regulations passed in recent years, and 2003, new ANSI Standards took effect. Formerly, both recirculating and 100% fresh air units were covered by the same standard. Today, ANSI Z83.4 applies to 100% Fresh Air Units and ANSI Z83.18 applies to Recirculating Units.

Typical components, options and accessories are listed below:

Mounting Curbs:

For installation on building roof. Note – As a service, some manufacturers will also specify mounting requirements for fabrication by others. This allows a contractor to field fabricate a curb for faster installation or allows for third-party fabrication of curbs where the engineer specifies seismic-resistant construction.

Inlet Hoods:

Generally an add-on item. Prevents rain and trash from being drawn into a unit.

Filters:

Filtration may be accomplished using Cleanable Filters mounted in the Inlet Hood or by using a Flat or V-bank Filter Section. For special applications, Bag Filters or HEPA final filters may be specified, but these are not common.

Clogged filter switches and indicators are often supplied to alert maintenance personnel when filters are dirty.

Fans:

Standard units are generally supplied with DWDI (Double Width, Double Inlet) Forward Incline fans. These are competitively priced and offer good performance characteristics. For High Static applications due to building pressurization, long ductwork, filtration, or other reasons, Backward Incline (BI) Fans may be required. Other options that may be specified are Backward Incline Curved (BC) or Airfoil Fans. These are BI fans with higher efficiency and performance, but at higher cost.

Insulation & Lining:

Most manufacturers offer the option of insulation and lining. Standard insulation is generally 1” thick fiberglass or foil face fiberglass board. Linings may be steel or stainless steel to isolate insulation from the air stream. This provides for longer-lasting insulation and eliminates the possibility of contaminating the air stream if insulation breaks down.

Gas-fired pressurization systems each offer good solutions to heating large structures such as warehouses, distribution centers, aircraft hangers, and manufacturing facilities. Situations that require significant ventilation favor pressurization air handlers as do facilities that generate dust, mists, and other contaminants. Cleaner environments with less intensive ventilation needs would do well to go with the infrared radiant heating approach. In very cold climates, it might be beneficial to go with a hybrid solution using radiant heat at the perimeter as the primary heating source and placing pressurization units at the center of the facility for ventilation and for pressurization when doors are open.

Additional information can be found at the NISCO web site http://www.nisco.net/fanblower.html.

Oleg Tchechel
Industrial Air Handling Application Engineer
NISCO Co.
http://www.nisco.net/airhandler.html
http://www.nisco.net/hvac.html