AxiTop – Less noise and more effi­ciency

For refrig­er­a­tion and cooling equip­ment, the heat gener­ated in the circu­la­tion process has to be trans­ferred to the ambient air via a heat exchanger. To accom­plish this, fans move cool air through a heat exchanger and allow the heat to dissi­pate. There is a whole range of options for designing and config­uring espe­cially effi­cient, quiet and durable fans. A new, passive compo­nent, the so-called diffuser, provides for a substan­tial improve­ment in effi­ciency and noise. Its pres­sure-boosting effect minimises exit losses and makes it easier to adapt the fan to commer­cially avail­able heat exchangers.
Venti­lation and air-condi­tioning units usually run in contin­uous oper­a­tion, or at least with long oper­ating cycles. That make maximum economy with the input drive energy impor­tant, for every addi­tional watt costs money and impacts the envi­ron­ment. For that reason, energy effi­ciency is an impor­tant crite­rion when choosing a fan. Today, statu­tory stip­u­la­tions also play a role. The effi­cient use of energy and resources is and will remain a global objec­tive for the decades ahead. In Europe, the first tier of the ErP Direc­tive became effec­tive as of January 1, 2013. Against this back­ground, it is no wonder that ultra-effi­cient EC tech­nology is increas­ingly replacing conven­tional AC tech­nology as the fan drive of choice due to its greater effi­ciency.

Exit losses – the under­es­ti­mated “energy guzzlers”

Figure 1: Power flow of an axial fan with unim­peded airflow

Energy-effi­cient and quiet oper­a­tion of the complete fan is a func­tion of both the motor and the impeller, which the fan uses to move the air volume needed to create the cooling air flow through the heat exchanger. Aero­dy­namic exper­tise is needed when designing the impeller, for example to avoid sepa­ra­tions and back­flows which would cause both energy loss and unwanted noise. Even today, the impellers employed in the Green­Tech EC fans satisfy the most demanding stan­dards. But there is another point which has to be taken into account when consid­ering the effi­ciency of a fan:
The inherent exit loss for fans with unim­peded airflow is often an under­es­ti­mated energy guzzler. Figure 1 shows the power flow of the input drive power P_o for an axial fan with unim­peded airflow. The drive power P_o is split into static blower output (P_us = product of the air flow and static pres­sure increase of the fan), which is of use to the user, and various losses caused by the conver­sion into this useful power. The largest loss factor in the process is the dynamic output compo­nent (P_ud) of air perfor­mance, which is also known as exit loss. This is made up of the product of air flow and dynamic pres­sure. The motor and fan manu­fac­turer ebm-papst has now picked up this issue and has devel­oped a new kind of diffuser, the AxiTop. Replacing the conven­tional guard grille of the fan by the AxiTop diffuser (Fig. 2) signif­i­cantly reduces losses at the air outlet. Effi­ciency increases while oper­ating noise is reduced at the same time. In prin­ciple, the diffuser works like a reverse nozzle, as follows:

Dynamic energy converted into static pres­sure

Figure 2: The AxiTop diffuser from ebm-papst provides for a signifi­cant improve­ment of efficiency and noise. Its pres­sure-boosting effect minimises exit losses and makes it easier to adapt the fan to commer­cially avail­able heat exchangers

Every medium is only able to absorb a certain amount of heat energy for each degree Kelvin. The possible temper­a­ture differ­ence and the amount of heat to be expelled define the cooling air flow required. This air volume has to be deliv­ered by a fan through the heat exchanger under consid­er­a­tion. To do this, a pres­sure differ­en­tial is neces­sary which is suffi­cient to over­come the flow resis­tance of the exchanger. Normally, the deliv­ered air flows at high speed from the exit side of the fan and dynamic pres­sure (p_fd) dissi­pates into the envi­ron­ment. Dissi­pa­tion means that the kinetic energy of the flow is converted into turbu­lence and then due to fric­tion into heat that is often no longer tech­ni­cally utilised. With the AxiTop diffuser, much of the dynamic kinetic energy is converted into static pres­sure (p_fs) by way of retar­da­tion. Phys­i­cally, this is easy to explain: The total pres­sure gener­ated (p_f) by a fan is the sum of the static pres­sure pfs and the dynamic pres­sure p_{fd_cges}. In turn, taking the density r into account, the dynamic pres­sure can be split into three speed compo­nents (cylinder coor­di­nates), the axial compo­nent p_{fd_cax}=ρ/2*c_ax², the circum­fer­en­tial compo­nent p_{fd_cu}=ρ/2*c_u² and the radial compo­nent p_{fd_cr}=ρ/2*c_r².

Figure 3: The AxiTop diffuser allows part of the dynamic pres­sure to be converted into useful static pres­sure by retarding the flow

In the diffuser, the axial and circum­fer­en­tial compo­nents of dynamic pres­sure (ρ/2*c_ges²) are reduced due to the air slowing in the expanding cross section and, due to the conser­va­tion of energy (Bernoulli´s prin­ciple), the usable static pres­sure compo­nent increases (see Fig. 3). Effi­ciency can be signif­i­cantly increased in this way if all compo­nents are co-ordi­nated to be aero­dy­nam­i­cally opti­mised.
In prac­tice, the use of the AxiTop diffuser does not just mean lower energy consump­tion; it also means greater degrees of freedom for the user and for the devel­op­ment engi­neer. The diffuser config­u­ra­tion can be opti­mised for different char­ac­ter­is­tics, depending on the appli­ca­tion concerned. Either a greater blower output is possible with unchanged energy input, or unchanged air perfor­mance is possible with lower energy consump­tion. A diffuser can also greatly improve the noise behav­iour. This acoustic improve­ment is espe­cially inter­esting when fans are working is a noise-sensi­tive envi­ron­ment, for example in overnight oper­a­tion in venti­lation and climate control systems in resi­den­tial build­ings or in rooms where people meet and where there are also noise protec­tion regu­la­tions to be fulfilled.

Signif­i­cant poten­tial to be exploited

Figure 4: Energy consump­tion and noise devel­op­ment are signifi­cantly lower with unchanged air perfor­mance. Blower output is increased and noise reduced with unchanged power input. (Details from an example appli­ca­tion)

The poten­tial of the possible energy savings, increased effi­ciency and noise reduc­tion provided by an optimal diffuser is substan­tial for common heat exchangers on the market. This has been confirmed in exten­sive test series. Figure 4 illus­trates a specific example. Replacing a normal fan with a guard grille with the same fan with a support bracket, guard grille and diffuser allows savings of 27 % for energy consump­tion while enabling an oper­ating noise level 7.2 dB(A) lower with the same air flow (see Fig. 4).
Alter­na­tively, if the greater effi­ciency of the fan with differ is exploited, it will deliver about 9 % more air flow with the same input power, and at the same time noise emis­sions will be reduced by 4.9 dB(A) (see fig. 4). This value has been deter­mined on an exem­plary customer appli­ca­tion. Depending on the indi­vidual config­u­ra­tion, the opti­mised effi­ciency can be used either to reduce power input or to increase air perfor­mance. So not only does the user save energy during oper­a­tion; the design engi­neer for a climate control system can get by with smaller heat exchanger surface areas. The space needed for the cooling unit can be reduced with unchanged or even improved noise behavior and constant refrig­er­a­tion capacity. The reduc­tion in the space needed is an argu­ment which cannot be neglected, above all for larger heat exchangers. A diffuser is even worth­while if it subse­quently turns out that the system does not have a suffi­cient refrig­er­a­tion capacity, e.g. in the case of a design error. It enhances the air perfor­mance without increasing noise. In such cases, the instal­la­tion of an addi­tional heat exchanger (and the asso­ci­ated costs) can often be avoided.

Can be retro­fitted and used in existing wall rings

Figure 5 (right): The thrust range is not affected by the AxiTop diffuser. The illus­tra­tion shows the axial flow rate with diffuser on the left and without diffuser on the right

Equip­ping existing heat exchangers with a diffuser is easy and only requires a few changes to venti­lation and air-condi­tioning units. The diffuser is fitted in place of the guard grille. Existing wall rings can still be used. The AxiTop diffuser from ebm-papst is just 250 mm high so it needs very little space.
With these dimen­sions, the devel­op­ment engi­neers have succeeded in achieving a very good compro­mise. Phys­i­cally, a diffuser cannot be large enough in order to increase static pres­sure. But dimen­sions which are as compact as possible are required in prac­tice. CFD simu­la­tions combined with para­metric opti­mi­sa­tions have produced an excel­lent aero­dy­namic result. The change in the outflow profile causes the flow to split slightly, enabling the thrust range of the fan to be main­tained. Figure 5 shows the different char­ac­ter­is­tics of the exit speed cax, which is of rele­vance for the thrust range, with and without a diffuser.
The new AxiTop diffuser is designed for fans of the sizes 800 and 910. Versions for 500 which are frequently employed with heat exchangers will follow in the near future. It doesn´t matter whether the fan works with Green­Tech EC or with an AC drive. However, in the interest of energy effi­ciency, the combi­na­tion with Green­Tech EC fans in certainly prefer­able.