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# Technical Info

Axair’s team of product sales engineers are able to advise on best practice and provide technical support regarding the selection and installation of the ECFanGrid.  To provide an insight into ECFanGrid’s technology, information is listed below on:

• How to determine spacing within an ECFanGrid
• How to set up a constant air flow control
• How to cover and handle a failure

To view the data sheets of the fans used within ECFanGrids, please see our backward curved centrifugal fans page and click on the product code.

## How to determine spacing within an ECFanGrid

One of the more common questions regarding the design of the ECFanGrid relates to the recommended space between the fans to maintain optimum performance. To answer this, measurements were carried out in the Rosenberg certified testing labs on the impeller diameter and the duct dimensions to see how performance is affected by plug fan spacing. As a result, the following ratio was established:

#### A/D > 1.55

In the ECFanGrid selection process, if this ratio or a larger value is achieved, there will be no negative effect on performance. The calculation of the ratio consists of two basic steps:

1. To establish the A value, both the x and y dimensions of the duct are divided by the number of fans in each direction.

2. The result must be divided through the impeller diameter D of the fan

A good rule of thumb is:

## How to set up a constant air flow control

Air flow with an ECFanGrid is measured by pressure taps in the inlet cone. Every inlet cone used with a Rosenberg EC Fan has a circular measurement tube with four measurement taps. The taps are arranged in order to obtain a correct mean value. This feature is critical if the air flow at the inlet is not even.

The pressure measurement is converted to actual air flow through the inlet cone. With the use of Bernoulli’s Incompressible Flow Equation and the Law of Continuity, the air flow is calculated by measuring differential pressure and applying a calibration factor. The actual measurement is the differential pressure between the inlet cone (measurement tube) and the inlet area of the application.

#### Air flow in the case of a failure

Shown in the graph below is an operating constant air flow control with a 3×3 ECFanGrid. As the curve indicates, the speed is changing and therefore the desired air flow is always maintained – 22.500 m3 /h (13,243 cfm). At the time t0, the damper was slightly closed and so the air flow in the system decreased. Hence, the ECFanGrid increases the speed to enlarge the air flow. The air flow reaches the desired 22.500 m3 /h (13,243 cfm) and the speed remains.

The ECFanGrid can be set up as a constant air flow system with a pressure controller used in 0-10 V open loop speed control as illustrated below. The desired set point is adjustable in the pressure controllers display menu.

## How to cover and handle a failure

Redundancy is a major aspect as to why fans are connected in parallel.

#### Detecting a failure

Every fan used in a ECFanGrid has a potential free alarm contact. The contacts are COM, NO (normally open) and NC (normally closed). “Normally” means the fan is powered and no alarms occurred.

One approach is to connect and evaluate the alarm relay. It is possible to get one terminal for no failure and another terminal for failure. If two LEDs are connected to the respective terminals, the green one will light when all fans are running and the red one will light if one fan fails.

If a PLC is used to control the ECFanGrid, the fans can be connected. Again if one fan fails, the Digital 1 will appear at the input of the PLC.

In most cases, it is enough to know that there is a failure on the site. However, it is also useful to know exactly which fan has failed from a remote location. Therefore, using ModBus RTU to evaluate the alarms is recommended.

#### Closing off the open cone of the defective fan

There are three possible solutions. The first is to leave the cone open until the fan can be repaired. In this option, there are large losses and a high probability that the remaining fans will not succeed due to their performance limitations. If, despite of the losses, it is decided to leave the cone open, it may be an option to set the remaining fans to maximum speed to maintain an emergency mode.

However, if it is decided to close the open cone of the defective fan, there are two options:

• Option A: Close off the open cone with a metal sheet
• Option B: Provide auto-backdraft dampers for all fans

Backdraft dampers would only be of value in rare cases of a fan failure. The main benefit of using backdraft dampers is that the cone is closed automatically, however they will cause some losses. In units under lab test, a simple, sealed metal sheet was used to close open cones. In addition, the metal sheet should be reinforced due to possible static pressures of 1.000 Pa (4 inches) or higher while an average sized metal sheet of 0,25 m2 (2.69 f t2 ) – the actual force acting on the sheet is 250 Newton or about 25 kg (55 lb).

With pressure or speed control, closing the inlet cone is all that is required. With the use of speed control, the superordinate system must increase the speed. Whereas in case of a pressure control, the failure will be recovered automatically, within the fan’s performance limitations.

#### Action to take if the master fan fails

The  solution is to change the master fan by using the measurement tubes from a different fan. This can be achieved by changing the wiring of the measurement tube. Therefore it is recommended to make all circular measurement tubes accessible from the outside. The air tube must be plugged from the master fan’s measurement tube to another fan’s tube.

Following a fan failure the ECFanGrid can be back in operation within approximately 15 minutes

For example, take a 2×2 ECFanGrid with constant air flow control 25.000 m3 /h (14,715 cfm) with a selection of covering one fan failure. The key for the selection is to check if three fans are able to maintain the operating point. When a failure occurs, it is detected by evaluating the alarm relay of the EC Fans, as discussed previously. At this point, the Building Management System is displaying an alarm and the duty officer goes on site to check which of the four fans has a malfunction. They proceed to close the cone by screwing on the reinforced, sealed metal sheet. If it is a master fan failure, they would additionally change the measurement tubes. The duty officer must also increase the set point to maintain the designed operating point. The black vertical line in the diagram below is the required air flow per fan. The demanded total operating point is 25.000 m3 /h (14,715 cfm), which in turn is 6.250 m3 /h (3,679 cfm) per fan. The intersection between this line and the blue curve is the regular set point, which should be adjusted in case of no failure. In this example, one fan failure would mean the number of fans is reduced to three. Now, the set point of the intersection between the black vertical line and the red curve (representing 3 fans) should be set. Finally, the duty officer changes the set point from 600 Pa (2.4 Inches) to 1.050 Pa (4.2 Inches).

If you would like us to specify an ECFanGrid for your application, please complete our retrofit enquiry form with the necessary details and send it to sales@axair-fans.co.uk. One of our team of product sales engineers will be in contact.