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Quotes - Engineering - SalesForced and Induced Draft
Where the Fan Sits and Why It Matters
Fan below the bundle pushing air up, or fan above the bundle pulling air through. It sounds like a minor detail. It decides air distribution, recirculation, fan life, maintenance access and how the unit behaves in bad weather.
Every air cooled condenser has to move a very large volume of air through a finned bundle. There are only two places to put the fan, and the choice has consequences that run through the whole design.
Forced draft puts the fan below the bundle, pushing ambient air upward through it. The fan handles cool air, so the fan, bearings, motor and drive all run at ambient temperature and last longer. Everything needing maintenance sits at grade or close to it, which makes service straightforward and cheap. Structural requirements are lighter because the fan and drive are not supported above the bundle.
The weaknesses are air distribution and recirculation. A fan discharging into a plenum produces an uneven velocity profile, with a fast core and slow edges, so parts of the bundle see less air than others. And because the air leaves the top of the bundle at low velocity, it tends to drift — and on a still day it can drift straight back down into the fan inlet. That recirculation raises the effective inlet air temperature, sometimes by several degrees, and derates the condenser in exactly the hot, still conditions where you most need it.
Induced draft puts the fan above the bundle, drawing air up through it. Because the fan pulls rather than pushes, the pressure profile beneath the bundle is more uniform and air distributes far more evenly across the face. Discharge velocity is high, so the plume is thrown clear and recirculation is much less likely. The bundle is shaded from direct sun and sheltered from rain and hail, and the unit is significantly less sensitive to cross-winds.
The costs are real. The fan runs in heated exit air, which limits material choice and shortens bearing and belt life. The fan, motor and drive sit above the bundle, so the structure is heavier and maintenance requires working at height. Fan power is slightly higher because the air being moved is hotter and therefore less dense.
A practical way to decide. If the plant is in a hot climate where recirculation on still days would hurt, or exposed to strong cross-winds, or where even distribution across a large bundle matters, induced draft usually earns its cost. If the process temperature is high enough that exit air would cook a fan, if maintenance access is a serious constraint, or if capital cost is the binding factor, forced draft is the sensible answer.
See horizontal fin fan and V-bank arrangements, or the air cooled overview.
Configuration
- Forced Draft: Fan below bundle
- Induced Draft: Fan above bundle
- Control: VFD, two-speed, louvers
- Fans: Axial, adjustable pitch
Features
- Variable speed control
- Adjustable pitch blades
- Louver control option
- Recirculation ducting
- Auto-variable fan hubs
Typical Condensing Duty
- Process vapor condensing
- Steam condensing cells
- Hot climate installations
- High wind exposure sites
- Cold climate freeze control
| ARRANGEMENT | FAN POSITION | AIR AT FAN | MAIN ADVANTAGE |
| Forced Draft | Below bundle | Cool ambient | Fan life & access |
| Induced Draft | Above bundle | Heated exit air | Even distribution |
Controlling an Air Cooled Condenser
Why Control Is Not Optional on Condensing Duty
An air cooled condenser is sized for the hot design day and spends most of the year with surplus capacity. On a liquid cooling duty that surplus is harmless — the outlet simply runs colder. On a condensing duty it is not, because condensing pressure is tied to condensing temperature. Over-condense and system pressure falls below where the process wants it, control valves lose authority, and in some systems flow through the condenser becomes unstable.
So an air cooled condenser needs a way to reduce its own capacity. Variable frequency drives on the fans are the most flexible and also save considerable fan energy at part load. Two-speed motors are a cheaper approximation. Staging fans off in a multi-fan bay is coarse but effective. Adjustable-pitch or auto-variable hubs change the air flow without changing fan speed. Louvers throttle air flow mechanically and are particularly useful in cold weather.
Recirculation, and How to Design Against It
Recirculation is hot discharge air finding its way back to the inlet. It is worst on forced draft units on still, hot days, precisely when the condenser has the least margin. The effect is a silent derate: the unit performs as if the ambient temperature were several degrees higher than the thermometer says, and no instrument on the skid reports the cause.
Design responses include raising the bundle to increase the distance discharge air must travel to return, avoiding tight clusters of bays that trap air between them, orienting the installation with respect to prevailing wind, fitting wind walls where cross-flow is a known problem, and choosing induced draft where the risk is high. On an existing unit that underperforms in summer with no other explanation, recirculation is worth investigating before adding surface.
Cold Weather Operation
In cold ambient conditions the problem inverts. Too much cooling can subcool condensate heavily, collapse system pressure, or freeze stagnant liquid inside tubes. Warm air recirculation ducting deliberately routes some discharge air back to the inlet — the same phenomenon that is a problem in summer, used on purpose in winter. Louvers reduce air flow, fans can be slowed, stopped or in some designs reversed, and internal recirculation can be arranged within a bay.
The important point is that freeze protection is a design feature rather than an operating procedure. Retrofitting it after the first cold snap splits a tube is expensive, and the arrangement of passes and headers that makes it work has to be settled before fabrication.
Choosing the fan side
Neither wins everywhere, so compare them on your own site numbers rather than on principle. A fan sitting in 150 F discharge air on an induced draft unit moves air that is about 9 percent less dense than 95 F ambient air, so it needs roughly 9 percent more power for the same mass flow and its bearings and belts run that much hotter.
Against that, a forced draft unit on a still 100 F afternoon can recirculate enough plume to behave as if the ambient were 105 F, and no gauge will tell you so. Weigh the maintenence side honestly as well: a motor change at grade is an hour with a forklift, while the same job at 25 ft above the bundle needs a crane and a permit. High process temperatures that would cook a fan in the outlet air lean toward forced draft; hot calm sites, windy sites and large multi-bay installations lean toward induced.
Recirculation is the fraction of warm plume air that is drawn back through the fan, and its cost is measured in lost driving force. If a process vapor condenses at 140 F against a 95 F design ambient, a 5 F recirculation rise cuts the available temperature difference from 45 F to 40 F, about 11 percent of the condenser's capacity gone on the hottest day.
You can measure it. Hang thermocouples at several points under the bundle on a still afternoon and compare them with the weather station reading; a spread of several degrees, worst at the center bays, is recirculation. The condition shows up in the plant as condensing pressure that climbs in the afternoon and recovers at dusk with no change in load. Raising the bundle, opening up the bay spacing or fitting an induced draft fan are the fixes, in rising order of cost.
Because a condenser with no throttle will drive the condensing pressure down until something else in the system breaks or stalls. Steam condensing at 15 psia sits at 213 F; let a winter morning pull the condensing temperature to 193 F and the pressure falls to about 10 psia, which may be below what a condensate pump or a downstream vessel is set to tolerate.
On a refrigerant or solvent condenser the result is a liquid line that runs too cold and a control valve forced nearly shut, where it hunts. On a vacuum duty it can be the opposite nuisance, with the condenser pulling harder than the vacuum system and air leaking in to fill the gap. A condenser rated for a 100 F design day has roughly twice the capacity it needs at 40 F, and fan control is simply the means of giving that surplus back.
Variable speed pays for itself through the fan laws: power varies with the cube of speed, so running at 80 percent speed uses about half the power and 50 percent speed uses about an eighth. That is why a VFD on a condenser that spends most of the year at part load usually returns its cost within a couple of seasons.
Two-speed motors give one step of that saving. Staging fans off in a multi-fan bay is coarse and leaves dead zones under stopped fans, which matters on a condensing bundle where a cold zone can subcool heavily or freeze. Louvers hold air flow back without saving fan power, which is why they belong on cold weather control rather than everyday turndown. One caution on VFDs: set a minimum speed so the motor keeps its own cooling and so the fan never idles in a stall region.
By making sure no tube can hold still condensate while the air across it is below freezing, which is a design problem settled in the pass layout before fabrication. On a site with a minus 20 F design winter, a tube that finishes condensing a third of the way along its length has 20 ft of liquid behind it sitting in air that can freeze it in minutes.
Warm air recirculation ducts and louvers hold the inlet air above roughly 35 to 40 F, fans run slow or reversed, and the first row that sees the coldest air is given the most vapor. Instrumentation is the other half: a low condensate outlet temperature alarm and a fan permissive tied to ambient temperature catch the condition before a tube splits. Startup is the riskiest moment, because vapor flow is low and the bundle is cold, so the sequence should bring fans on only after the headers are warm.
The design ambient temperature and how still the air is on those days matter most, because they decide whether recirculation would eat the margin. A site with a 100 F design day and light summer winds favors induced draft; a site with a 90 F design day and a steady breeze can run forced draft without penalty.
The process vapor temperature comes next. Discharge air leaving at more than about 200 F shortens belt and bearing life on a fan sitting above the bundle, which pushes the decision back toward forced draft. After those, plot spacing to neighboring bays, prevailing wind direction relative to the bank, whether the plant will accept work at height, and the winter design temperature for freeze control each move the recommendation one way or the other. Noise limits favor induced draft slightly, since the bundle sits between the fan and the ground.
