There are two main types of evaporative condensers: forced- draft and induced-draft.
Of the forced-draft type, there are centrifugal fan models and axial fan models. As for the induced-draft, axial fan models are the most common in today’s market. Each product has certain features that impact the decision to choose one type of condenser over another for a specific application.
The forced-draft axial fan condenser is the most common condenser in the field. Its axial fan design provides for low motor horsepower and the location of the mechanical equipment offers easy access. This product is available in most capacity ranges and many different footprints. It is more often used in industrial applications or for large-capacity installations.
The forced-draft centrifugal fan model is more often used in halocarbon air conditioning applications (small-tonnage installations and ice rink applications where sound may be an issue). The centrifugal fan design makes it the quietest condenser available.
The use of centrifugal fans also allows for this unit to be installed indoors or to be provided with optional sound attenuation or high-velocity discharge hoods. The drawback for this design is that the energy consumption is roughly twice that of axial fan designs.
Induced-draft axial fan models, introduced in the mid-1980s, are being used on a large percentage of projects, regardless of size. The axial fan design provides low energy consumption similar to the forced-draft axial fan models.
The modular design of induced-draft units provides for ease of maintenance — a real benefit to end users. The mechanical equipment is mounted near the top of the unit to draw air through the condenser.
Much has been written about the benefits of each type of condenser. Each product has its own merits. Product choice needs to be based on factors such as familiarity with the equipment, the commonality of parts for an existing installation, the physical location of the condenser, and both the installed and annual operating costs of the condenser.
In order to understand the differences between the types of condensers, let’s compare product features in terms of sound ratings, air recirculation, and equipment maintenance.
In the forced-draft design, ambient air enters the unit at the base and exits as saturated air at the top. The air is forced or pushed into the condenser, creating a positive pressure inside the condenser. This increases the potential for water leaks in the basin and casing sections.
Induced-draft models draw or pull ambient air through the condenser at the base, thereby creating a negative pressure in the casing and minimizing the leak potential.
In some cases, sound may be an issue. Moving air and falling water create sound. Therefore, sound can be a determining factor in equipment selection.
The level of sound is directly affected by the proximity of the sound source. Therefore, the location of the equipment with respect to neighbors should be taken into account when selecting condensers.
Induced- and forced-draft condensers have very different sound levels. The fans on a forced-draft condenser are mounted at the base of the unit. Sound propagates outward from the condenser in waves horizontally.
The induced-draft condenser directs more of its sound power vertically because the fans are mounted at the top of the unit and the air (and, therefore, the sound) is directed upward.
A comparison of sound ratings for the three types of condensers illustrates the difference in fan location. The forced-draft axial fan models have a much higher decibel level from the air inlet, while the induced-draft models are much closer to the centrifugal fan models.
The centrifugal fan model has the lowest overall sound levels, with the induced-draft model second and the forced-draft axial fan loudest. Depending on the installation, the directionality of the airflow and sound propagation can relate directly to the type of equipment selected.
Another aspect of equipment selection is recirculation of air. Recirculation is the introduction of saturated discharge air into the air intake of the condenser. Air recirculation dramatically reduces condenser performance.
The forced-draft condenser has a relatively low discharge velocity and a much higher entering-air velocity. The saturated discharge air has a greater possibility of being recirculated.
The exiting-air velocity of an induced-draft model is much higher than the entering-air velocity. The leaving airstream is more likely to be dissipated as it travels upward away from the condenser.
In terms of maintenance, the number of components such as belts, pulleys, and bearings that must be maintained directly impact the amount of time required and thus the total cost for maintenance. For a given capacity, the induced-draft model will have fewer components than the equivalent forced-draft model. Fewer components will lead to less maintenance time for the service technicians and result in lower operating costs.