Airedale heater issues

[Kirky]

Hi all hope everyone is wellGot a issue with a Airedale close control AHU. The unit is a Airedale V9CW indoor AHU providing close control of temp and humidity for a laboratory. Cooling is supplied via an outdoor Airedale chiller, humidity via a humidifier cylinder located in the indoor and heating via 3 elements located at the back of the indoor coil. Simple setup which I'm sure your all familiar with, so I won't go on. A few months ago I was called out as the unit was not heating. Found that 2 of the 3 elements had blown. Replaced all 3 and restarted the unit all ok. The unit ran fine for 4 days and then alarmed on its heater overheat. Couldn't see anything wrong with the heaters themselves and they are a direct replacement from Airedale. Reset the overheat cutout unit ran fine for another 4 days before tripping again on overheat. Checked the supply voltage 433v says on badge that unit is 3 phs 400v. Does anyone know what voltage these heaters are rated to?. This system has been in operation for over 10 yrs. Customer tells me that the elements blow and need replacing annually. But the overheat cutout every 4 days is new. I have suggested that the supply voltage and current be monitored to highlight any spikes which could be causing this issue. I have also the installation to ensure the overheat isn't touching the elements all looks ok. Is there anything else which I am missing any advise welcome

[bigor_2]

that is triggered? overload relay or safety thermostat?

[Brian_UK]

Check the airflow?

Also, is the overheat device old and failing. Simple bemetalic ones have a limited lifespan.

[frank]

What is the heat output rated at and what amp draw are you seeing on full load?

Air on and air off temps?

[bigspee]

Overheat stat faulty? Poor air flow?

[hyperion]

Has the increased three phase voltage been a new change on the site? 433v three phase equates to approximately 249volts single phase. Most of the heater elements output are rated at 230volts, so whilst the heater will not be instantly physically damaged by the over voltage, it will significantly increase the surface temperature and therefore reduce the longevity of the elements, hence the need to change the elements more frequently.
If there is anything causing a reduced airflow, with the higher surface temperature mixed in with a tired overheat cut-out, this could be the cause.
Check the fan speed, drive belts, return air filters, air distribution ductwork, grill louvres. Anything that may have changed.
Suggest that you replace the overheat cut-out for starters.

[sedgy]

clean filters ?= sedgy, have you found any fire dampers? if its overheated the fire link might have melted = result fire damper shut ?
sorry has anyone got ant tipex ¿

[sedgy]

hi , if the unit has overheated and if there is a fire damper in the ducking , the fire link might have melted , resulting in the fire damper ( spring loaded) shutting.
this F/D is normally directly after the heater box onto the supply duckting,
regards = sedgy

[Kirky]

Hi guys sorry for the late reply was sent on another job. Anyway heaters are 2.5Kw each and we have 3 of which is single stage heating. Checked the voltage and current draw again today. 435 436 437v 11.5 A current draw per phase fully loaded. I have checked the airflow filters are clean and coil is clear. The Drivebelts are on the slack side. Customer has advised that this has always been the case as when properly tensioned airflow is too noisy for lab techs. On arrival today the unit had not tripped since Monday morning and has remained in spec with no problems. The unit was just switching modes from cool to heat air on was 19 degC off steadily increased from 15deg to 28deg during heating cycle. It took approx 4-5 mins to hit setpoint before switching modes back to cooling air on was 24 DegC off decreased from 28 to 14 degC before hitting setpoint again. I have watched the unit control switching modes for a good hour without issue. When it does trip it triggers safety thermostat and not overload relay. The only thing I haven't yet seen is the unit heating and humidifying at same time. I am returning to site later this week to force this to happen. As far a fire dampers etc there isn't any and the airflow is not restricted. I will update tomorrow. Cheers for all responses

[frank]

I would take a hair dryer in and warm up the overheat sensor to see what temp it is switching at.

Your current draw is slightly high (0.6A) indicating that the element is running a bit hot.

The spec sheet for the unit says that is has a nominal air volume of 0.65m3/s.

With a 7.5kW heater, the maximum temp lift should be around 9 - 10 K. You are measuring 13K.

It all points to a low air volume.

Looking at the installation manual, there is the option to set the fan speed in the controller.

here's a link to the manual..it's on page 79 and 80...scroll down to the "Precision" 3rd one down

[hyperion]

Would recommend that the fan drive belts be correctly tensioned and then if need be, the fan speed adjusted as Frank has suggested.
The slack belts could be reducing the airflow by an unknown amount and this would then be the cause of the overheat sensor tripping out.
Would also suspect that there is insufficient heat load in the lab and the unit may be trying to control the conditions via the humidity sensor and then dropping the dry bulb temperature too rapidly if the system is cycling heating/cooling/heating.
The reduced airflow may also be also be affecting the RH% in the lab.

[Brian_UK]

^^^Agree with above comments, in the absence a working fan speed control then consider changing the drive pulleys to achieve the required fan speed without belt slip.

It's more economical in the long run.

Try also selling them thyristor control of the heating elements.

I wonder as well with having said that the lab staff don't like the proper airflow - has someone been turning the unit off when in heating mode? No airflow would no dissipate the residual heat causing a trip.