When you read the specification sheet for an air-cooled heat exchanger, typically it is the “worst case” operating condition, with adjustments made for the degradation in performance expected over the unit’s lifetime. The design case can be as simple as the highest expected ambient air temperature, or it can include several different operating conditions at different air temperatures.
Some users further complicate matters for themselves by insisting that a unit be purposely over-surfaced by 10 percent or be sized using a duty 10 percent more
Ways to Deal with Off-Design Problems - jump to:
Warm Air Recirculation
Two-Speed Fan Motors
Fan On/Off, Multiple Units
Variable Frequency Drive
Auxiliary Heating Coils
than the process being cooled. In tandem with the fouling factors, this can result in an air-cooled heat exchanger that will have excess surface areas typically between five and 20 percent higher than what is nominally required to meet the peak performance condition. Excess surface area often results in a heat exchanger that overcools the process.
In real life, the air-cooled heat exchanger will seldom operate at the design condition. In fact, the highest expected ambient air temperature usually will only occur less than two percent of the time. Therefore, 98 percent of the time, the ambient temperature will be less than that listed on the data sheet. The reality is the two percent is an overstatement; remember, the heat exchanger, in the “as new” condition, is actually designed to exceed the performance due to the fouling factors applied. This means that all air-cooled heat exchangers spend little or no time actually operating at the design condition specified.
In real life, the air-cooled heat exchanger will seldom operate at the design condition. In fact, the highest expected ambient air temperature usually will only occur less than two percent of the time.
With all of this in mind, in the best case scenario, a unit operating at an off-design case will simply overcool the process fluid. In the worst case, that lowered process fluid outlet temperature could be below the process fluid’s freezing point. Suppose that fluid is water. Water can expand and destroy the tubes in a matter of minutes. Or, the outlet temperature could be below the pour point of the process fluid, leading to solidification within the tubes. This creates a cleanup issue. Or perhaps the fluid is sub-cooled below its saturation point, requiring pre-heating somewhere else in the process at additional energy expense. In fact, in some applications, the operator simply cannot live with the large swing in outlet process temperatures — the impact to their downstream process is too high. Instead, ways to mitigate and eliminate the temperature swings are needed.
Ways to Deal with Off-Design Problems
Remember, the controlling side on air-cooled heat exchangers is always the air side. Typically, adjustments are not necessary on the process side (bypass valves or otherwise). Luckily, all air-cooled heat exchanger manufacturers have several ways to help users maintain control during the day-to-day variances in ambient air temperatures. They include:
• Adjustable louvers.
• Warm air recirculation.
• Two-speed fan motors.
• Fan on/off on multi-fan units.
• Variable frequency drive.
• Auxiliary heating coils.
Adjustable Louvers. Adjustable louvers vary airflow by increasing or decreasing the air-side flow resistance. They can be manually or automatically actuated. Manual actuation typically is used when the variance in air temperature is seasonal and only requires adjustment a few times a year. Many air-cooled heat exchangers used on compressor stations utilize manual louvers.
Automatic louvers typically are pneumatically actuated; that is, opened or closed upon the pneumatic signal (figure 1). Some may have fine adjustments and may be tied to sophisticated control systems that use process temperatures to drive the degree of openness of the louver.
Louvers are simple to maintain and ideal for control in many situations. Their main drawback is that the fan power usage remains constant during their use. In fact, the very existence of the louver — even in the open position — introduces additional air-side resistance and will result in extra power usage by the fan to maintain the same airflow.
Warm Air Recirculation. There are some situations where the control provided by the louvers is insufficient, especially in cases of extremely low inlet air temperature. In those cases, it is helpful to recirculate some of the heated outlet air back into the inlet side (figure 2). Typically, the louvers are controlled automatically and are tied into the plant control system. The main disadvantage to warm air recirculation is that ductwork must be extended completely around the air-cooled heat exchanger, typically raising the cost of the unit by 20 to 30 percent. Once again, the louvers and ductwork will result in extra power usage by the fan to maintain the same airflow.
Two-Speed Fan Motors. An alternative to the use of stand-alone louvers or warm air recirculation systems is the use of two-speed fan motors. They are AC fan motors that have been wound for two different speeds of operation. For example, a motor may be rated for both 1,750 and 875 rpm operation.
Unlike louvers, the two-speed fan motor will reduce power consumption when operated in the lower speed category. Operating at the lower speed also will reduce noise. The main drawback to the use of two-speed fan motors is the added expense of the second winding. Also, similar to the stand-alone louver, two-speed fan motors cannot be used to increase the air inlet temperature like warm air recirculation.
Fan On/Off on Multiple Fan Units. A variation on the two-speed fan concept is to completely turn on or off individual fans on multiple fan units. Once again, power consumption and noise can be reduced as fans come on and offline. In the case of all fans being shut down, heat transfer will still occur due to natural convection. The main disadvantage to this method is the need for individual control of the motors and the additional logic that must be built into the plant control system.
Variable Frequency Drive. The cost for variable frequency drive (VFD) technology cost has come down considerably in the past few years. The drive unit varies the frequency of the AC power being supplied to the motor to allow it to run above or below the standard 60 or 50 Hz. This allows much more fine-tuning than two-speed motors or turning fans completely on or off. Most electrical motors can be rated for VFD operation at little or no additional cost.
Auxiliary Heating Coils. There are situations where warm air recirculation may not be sufficient to keep a coil from freezing. When immediate heating is required, heat can be provided on-demand by running either steam or heated glycol through a heating coil that is adjacent to the process coil (figure 3). While this solution can provide immediate heating and quick control, it is the most complex and expensive of the solutions presented.
While it is obvious that an air-cooled heat exchanger must be designed for the “worst case” conditions for maximum performance, many techniques are available to allow for control over the entire off-design range of the unit. Consultation with the air-cooled heat exchanger manufacturer early in the process will help decide the most economic solution for a given process design.
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