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We have 2 Emerson Liebert air conditioners inside a datacenter. Issue is not a home issue but devices are all same and I supposed finding some expert here. Here is the thing:

This 2 devices are designed to work together, communicating via network cables over ethernet switch. Both has Hiross compressors, one has an additional unit named Siemens Poly Cool Superheat (I am not sure but it might be something like Variable Frequency Drives). Our mechanic said its compressor is also different than other's.

This device was broken for over a year. We have had a mechanic fixed the device today. Everything seemed normal while the mechanic was at ours. Later, I have noticed something weird. Condenser's fan runs at a very high rpm than interrupts in 2 or 3 seconds. Then, just it is almost stopped, runs again at a high rpm. It continues like so. Even room temperature was over than set value, it runs like so.

Is there anyone knows about Hiross or Siemens Poly Cool module or Emerson Liebert air conditioners?

  • I am guessing what you are calling a super heater is a 'water super-deheater,' a clever idea that produces hot water 'for free' when the compressor is running. It is nothing but a heat exchanger from hot side of the refrigerant line, to the hot water supply. – Some Guy Mar 28 '15 at 22:28
  • There is no water installment. It operates with R407C gas. The mechanic was not sure about the module but I think it controls the compressor to work more efficient. I am not sure. – Kemal Mar 28 '15 at 22:31
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    What did they do to 'fix' it? – Mazura Apr 1 '15 at 1:06
  • Luckily they just filled up refrigerant gas. Before them, another mechanic saw it and said compressor was broken. We decided to call for someone else and they said its compressor is different. Because of PolyCool module it works like it was broken. I think fans abnormal behavior is also because of PolyCool optimization. – Kemal Apr 1 '15 at 20:12
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Depending on your specific equipment setup, you may have a variable speed condenser fan, variable speed compressor, or both. With a variable speed compressor and condenser, the system would ideally maintain stable superheat and suction pressure (effectively controlled by the Siemens superheat controller) and would maintain a stable temperature differential across the condenser. If the compressor modulates to meet the demand on the coil, the discharge pressure should remain reasonably stable.

I have seen in refrigeration and HVAC that a technician will often override the compressor and/or condenser modulation (either VFD or ECM controller) and put them "in hand" or into fixed speed mode. The first thing you should check is to see if the controllers are able to modulate speeds; if they have been put into override, set them back into auto control.

Whether the compressor is variable speed or not, if the compressor is being cycled on/off too rapidly and the condenser controller settings are off, it may be driving up and down rapidly to accommodate the fluctuating load from the compressor. It may also be tripping on current overload, as already mentioned in another answer. The following assumes that the condenser fan is not having electrical issues.

Observe the compressor behavior in conjunction with the condenser's cycling: is the compressor cycling off and on with the condenser? If so, verify that your compressor controller settings are not trying to adjust to changing suction pressure too rapidly (slow down the ramp time, increase the proportional band, etc.) Regardless of whether the compressor is fixed or variable speed, it should be running for 5-10 minutes at a time (at a minimum) in cold weather; any more frequently and the compressor is short cycling.

If the compressor is short cycling, correct this and verify that your condenser controls are no longer having issues.

If the compressor is not short cycling (or if you have fixed the issue and are still having condenser fan problems), take a look at the condenser controller settings. Verify that the set points are reasonable (i.e. a 20-30F, sometimes 40F temperature differential over your condensing medium is typical for HVAC systems; refrigeration typically runs between 10-20F). Also check the controller settings (proportional band, ramp time) for the condenser cycling. Like the compressor, the condenser should be running for longer periods of time, but it will not be running during extended periods when the compressor is off.

Verify your evaporator settings, too. The Siemens superheat controller should be trying to maintain a specific leaving air temperature temperature (typically 55F) with a temperature differential across the coil (anywhere from 5-30F) with 10-20F superheat at the outlet. When the thermostat sends a cooling signal to the unit, the compressor and condenser should cycle on, and the superheat controller should maintain a stable evaporating temperature.

If you are still having issues after verifying that the controllers are set correctly, begin checking the temperatures and pressures. Verify that the condenser pressure is somewhere between 20-40 degrees F over your ambient outdoor temperature (assuming the condenser is outdoors; if it is condensing in another conditioned space for some reason or using a fluid cooler, substitute that temperature for outdoor air temperature). Use a refrigerant pressure-temperature chart (US, SI) to check your discharge pressure versus the condensing temperature. If your ambient temperature is 90F and you have a 10F temperature differential (TD), your discharge pressure should be around 225 psig from the compressor (which corresponds to 100F). Do the same for the evaporator: the evaporator inlet pressure should be approximately equal to the condensing pressure and the evaporator outlet pressure should correspond to the evaporating temperature. With a 10F TD across the evaporator (i.e. 45-55F), the evaporating pressure should be around 70psig. The evaporator outlet temperature should be in excess of this value; the difference between the temperature that corresponds to the evaporator outlet pressure (measured using a transducer or pressure gauge) and the actual evaporator outlet temperature (measured using a temperature sensor) is known as the superheat. The superheat should match the commanded superheat on the controller.

If you are still having issues after verifying all of the above, post back with additional information about the system (model numbers) and the pressure/temperature readings from the controllers.

One final thing to consider: R-407C, being a blend of different refrigerants, will exhibit what is called "glide." What this means for you is that at a given pressure, you can have two observe two different temperatures (dew and bubble) depending on how much of the refrigerant is liquid and how much is vapor. This can cause problems with your controllers if they are expecting a specific temperature/pressure setting, e.g. you have P=20psig at the bubble temperature (when the liquid begins to vaporize) instead of the dew temperature (when the vapor begins to condense to liquid). The P-T chart I linked to above indicates whether the number is a dew or bubble temperature. The concept makes more sense when you look at a pressure-enthalpy diagram (I can't post a third link; Google "Dupont R-407C pressure enthalpy diagram"). Note that inside the saturation dome, the isotherms (lines of constant temperature) have a slope between the left and right sides of the dome. For single-component refrigerants, the line is flat/there is no glide.

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    It seems the issue was as you said Ryan. The abnormal operation is not a fault but normal act. As I found out, PolyCool controls both compressor and the condenser fans for energy efficiency. If heat is too high, it signals compressor and fans to work by full capacity. Since it reaches to set point, it reduces the capacity usage and saves energy. Something like that... Thank you – Kemal Apr 1 '15 at 20:21
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Former HVAC controls guy here - so I understand this stuff a little bit. Disclaimer: It has been about 10 years since I worked on this stuff so my memory is a bit rusty.

The Siemens module is controlling the amount of evaporator superheat by means of modulating the TXV (thermostatic expasnion valve). So the mechanic is partially correct, it does make the entire system run more efficiently - not just the compressor.

However, this should really have no bearing on the condenser fan tripping off. It sounds more like the condenser fan is pulling too many amps and tripping the overload, the controller has gone bad or has faulty inputs causing abnormal behavior. Not sure how the condenser fan control scheme works, but also check the indications from either pressure or temperature sensors that are used by the fan controller. Good luck!

  • Thank you for your answer kkeilman. I think it's because Ryan's suggestion. – Kemal Apr 1 '15 at 20:17

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