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Don’t Err With Fans

May 5, 2020
Understand the tradeoffs posed by the various options

To move large volumes of air at low differential pressure, plants rely on both forced-draft (FD) and induced-draft (ID) fans. The former push the discharge air from the fan into the process while the latter pull suction air to the fan from the process equipment. Sometimes, FD and ID fans work together in the same equipment; a common example is air supply to the combustion side of a fired heater using both types.

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Process/fan interaction and fan mechanics determine the choice of FD versus ID fans for specific situations. Generally, FD fans have backward curving blades while ID fans have forward curving ones. Most fans use constant speed motors and vary capacity with either louvers or variable pitch blades. However, controlling capacity via a variable frequency drive (VFD) to adjust motor speed usually is best. Modern VFDs often are highly cost-competitive in spite of their higher initial purchase price (VFDs frequently are much more reliable and cheaper to maintain).

At a constant speed and fan diameter, an ID fan produces a higher speed of air leaving the blade and a greater discharge head than a FD fan. However, at constant capacity, the aerodynamic disturbance from the forward curving blade of the ID fan also creates more noise and vibration. Large ID fans can be very noisy. While efficiencies vary with specific fan shape, ID fans generally are less efficient and require more power than FD fans for the same pressure rise and flow rate. Nevertheless, because of its higher air velocity, the ID fan is favored for large-capacity low-head systems.

Either type of fan can be placed in elevated structures. However, the increased vibration from ID fans will raise structure costs for fans at high elevations.

The forward lean of the blades on an ID fan tend to capture sticky solids more readily than the blades on FD fans. Thus, ID fans may require more frequent cleaning to sustain efficiency and capacity and to avoid balance problems.

While an ID fan will have a smaller diameter for the same capacity and head, its fan assembly normally will cost more. The extra expense comes from the need for a larger motor (because of the fan’s lower efficiency) and stronger fan components (due to vibration, power and balancing).

Used by itself, an FD fan pressurizes the downstream process. So, air may leak out of the equipment. For a cooling tower, this only may be a minor issue. However, an FD fan upstream of a heater firebox can lead to leaks of high-temperature combustion gas that can damage equipment and pose a safety problem. Heater fireboxes subject to positive pressure require excellent refractory and firebox enclosure maintenance to prevent leaks.

In contrast, an ID fan drops the process pressure below atmospheric pressure. Leaks go into the equipment. On fired heaters, inward leaks of air decrease efficiency because excess air becomes difficult to control.

A so-called balanced-draft system includes both FD and ID fans. Common on fired heaters and solids drying systems, the balanced-draft configuration allows regulating pressure in the equipment to minimize air leaks in either direction and enables good overall flow control.

Some heaters can operate under natural draft without any fans at all. This may require a reduced firing rate. Other heaters must shut down if the fans don’t run. The stack may not have enough height to generate draft or mandatory pollution-control equipment imposes such a high pressure drop that operation is not feasible without fans. The ability to run under natural draft is an important consideration in shutdown systems.

For a heater with only one fan service, fan failure causes a heater shutdown or a shift into natural draft operation.

Balanced-draft systems require safety features to allow safe operation or shutdown if one fan motor fails. Most often, the heater has both high-pressure and low-pressure shutoffs. If the ID-fan fails, the high-pressure trip may shut down the furnace completely or shut off the FD fan to prevent the heater from over-pressuring. If the FD fan fails, the low-pressure trip may shut down the furnace completely or shut off the ID fan to prevent the heater from going into too much of a vacuum.

Of course, shutting down the second fan but keeping the heater running only is practical if the heater can operate with natural draft.

ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at [email protected]

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