Instructions to change a Fan Tray

Each DIRC Wiener CRate (WCR) hosts an fan tray whose front panel is visible in the following picture, below the boards (1 DCC on the left + 12 DFBs on the right; 2 DFBs are missing on this picture to complete the crate). One can see the ON/OFF red button on the right, the 2 black screws (top left and right) fixing the tray in the crate and the digital screen allowing to setup the different parameters of the fan tray. There are 9 fans per WCR, hence 108 for the whole DIRC.

Fan tray of a Wiener crate
Front view of the WCR with the tray panel visible in the lower part of the picture.

The two next pictures show the bottom view of the fan tray (with the grid protecting the 9 fans from big dust particles) and its inner side where the fans are visible.
The only fiability problem we've had with the WCR is some fan failure due to the ageing of these components. By default, when a fan fails, the crate automatically switches off which makes BaBar obviously non-runnable and makes us loose some data. 6 of the 9 fans are connected to epics which allows their status to be bypassed and hence the crate to work even if one of these fans is faulty. Unfortunately, the 3 other fans (added on purpose to ensure a better cooling of the crates) are not connected to epics. If one of them fails, the only solution is to change the fan tray!

Fan tray of a Wiener crate
Bottom view of the fan tray. Behind the grid protecting them, one can see the 9 fans: 6 on the right connected to epics, 3 on the left, added specially to cool the DIRC crates and not connected to epics.

Fan tray of a Wiener crate
Inner side of the fan tray with the fans more visible: the 6 connected to epics on the left, the 3 not connected on the right.

Changing a fan tray

Although changing a fan tray may seem pretty straightforward, this operation must be handled with care. If you have some free time before accessing the detector, trying the procedure on the spare WCR located in the DIRC teststand is definetely a good idea. To properly change a fan tray, the following steps have to be performed.

  1. Print, read and sign the non-routine JHAM dealing with the work on WCR.

  2. If needed, wear an harness and attach it safely -- check with safety people (the "green hats") if you have any doubt.

  3. Unscrew the two lemo connectors (one male, one female) which ensure the communication between the crate and epics.

  4. Slowly unscrew the two black screws on the tray front panel (top left and top right). While unscrewing, you should regularly alternate between one screw and the other. There is no need to remove them completely: at some point you should feel the tray ready to move.
    Be careful: only these two screws hold the tray in the WCR! As the tray is on two rails, it cannot fall outside the WCR but it can start slipping along it, especially if the crate is not horizontal.
    Controlling this motion is indeed the best way to remove the tray. After a path of a few centimeters, the holes inside the tray match those on the rails which allows one to remove the tray outside the WCR.
    Be cautious: the tray is heavy, especially if you're on a big ladder.

  5. Put the new fan tray in place by following the steps above in the reverse order. Then, you should configure it to make it working. To see the different settings, use the MODE SELECT button at the left of the digital screen. The value of the selected parameter can be changed with the ADDR button -- the second button from the left on the panel.

Fan alarms

In each WCR crate, alarms with quite stringent thresholds have been set on the average speed of the 6 fans monitored in epics. This should allow us to discover a weaker fan at least few hours before it fails (its behaviour becomes usually erratic before it stops). If such alarm occurs, the DIRC commissioner will be paged immediately: among other things, one has to make sure that the crate temperature remains low enough.

Fan alarm on ALH
Fan speed alarm for the WCR in sector 0 in the Alarm Handler:
DRC → DRC_WCR → SECTOR_X (0 ≤ X ≤ 11) → DRC:SC_X:WCR:FAN:S
The nominal fan speed value is 3120 rpm, the yellow alarm is at 3050 rpm and the red alarm at 2950 rpm.

In addition, the fan failure signal is disabled by default on the WCR: tests have shown that the WCRs are correctly cooled with 8 fans only and so there is no need to interrupt the data taking if one of the fans monitored in epics fails. In any case, the problem will appear on the Alarm Handler and so the DIRC ops. crew will be aware of it. There is no urgence to change the tray: this can wait for the next door opening.

Wiener crate epics panel
Epics panel for the WCR in sector 0. The current fan speed is compared to the nominal one in the bottom right chart (the two arrows should match) and the buttons to enable/disable the fan failure is on the left. By default it is disabled.

On the other hand, if one of the three fans not visible in epics fails, the crate will be switched off automatically. In that case, the only way to fix the problem is to open the proper backward door and to change the fan tray. Call immediately the RC (and the Technical Coordinator) to warn them and to setup the opening asap.

During the October 2005 shutdown, all fans will be changed to prevent such failures in the future. Hopefully, we should not get any fan failure after this reparation as fans have been working more than three years without problem after the BaBar startup.

Web Page created by Nicolas Arnaud
Last updated on June 27th, 2005