Recommendations for actuating a 30 kW heater 5

[amazing azza]

Friends, I have an in-duct electrical heater that is part of a dehumidification and cooling system. The cooling coil first brings the air temp down real low, to drop out as much water as possible, and the re-heater then brings it back up to a comfortable level. The AC would run at a fixed setting, while the actual temperature setting would be done via the heater.

The heater is a set of 2x15 electrical-kW in-duct heating coils. It is to be actuated by an ON/OFF temperature controller.

How would you go about turning it on? Regular motor contactor? They don't make solid state relays that big, do they?

 

[roydm]

Usually electric heating is controlled via a solid state time proportional power controller.
This cycles the power On/Off over the period of a few seconds depending on the output from a temperature controller delivering complete half cycles. (the tyristors are switched on as Voltage passes through zero)

30 Kw is not a large controller

If you Google "solid state heater controller" it will pull up a couple of the vendors.
For a 30 Kw heater you will need a 3 phase controller with 4-20 mA input, sometimes they switch just two legs. You will also need short circuit protection (fuses or breaker)

 

[itsmoked]

Besides the phenomenal waste of power this scheme requires after following Roy's good info you also need to include safety stuff as heater elements in a duct are setup to burn down facilities. You need to include a safety contactor and over temperature sensor(s). Whatever you use to control the power to the heating coils can fail ON. If it does and the air stops...

So, you include a backup mechanical contactor that is run with a lock-out that needs human intervention to re-enable.

Alternatively, do it right by following the dewatering evaporator coil with part of the condenser coil of the same system. Use the extracted heat you've already paid for to reheat the air, not more electricity you have to buy. Furthermore, this negates the safety stuff you'd have to otherwise purchase and install.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

roydm, thank you! That is exactly the info I was looking for.

itsmoked, yes, you are absolutely right - the wasted power is the first thing that came to my mind when I saw this system as well. However, it already exists (don't ask) and I must do something about it. In fact, initially I wasn't even going to use the re-heaters. But... the system is *grossly* oversized and I am having problems with it choking on its own power. As the return air temp drops there is not enough cool being removed from the evaporator coil which leads to TXV throttling off the refrigerant flow until the compressor trips on internal overheat controls. I have to find a way to remove more cool from the evaporator coils.

One way would be to increase the air flow across the coils, but the small-ish room that is on the receiving end of this flow starts to vibrate. Not good. The other way is to increase the return air temperature. I have tried admitting outside air to this effect, but this results in a drop of suction in the return air ducts - and the room becomes overpressured. Additionally, there are fancy filters used for the supply air, which will not last long with against outside air. But then, there is that re-heater coil... already in place, collecting dust. It would be the quickest available solution, as long as it is done safely.

 

[thebard3]

I have seen this scheme in large office buildings, built during the '70s and '80s that have no regulation on the cooling, therefore the AC runs all the time at max output with duct heaters on each zone. Then all the office workers still have a space heater under their desk to keep them warm when it's 95 degrees outside. I guess electricity was cheap then.

Brad

It's all okay as long as it's okay.

 

[amazing azza]

yikes, and here I thought I was being wasteful!

Having a hard time finding the thermal switch... itsmoked, did you mean to read a thermal sensor from a plc, or some kind of a mechanically resettable temperature relay?

 

[itsmoked]

Hi Azza,

To carry out the safety aspect you need to interrupt that back-up contactor's coil somehow. You can do a bunch of different ways.

Note: in no way am I endorsing any of these first-example-I-found links. Find your local equivalents.

1) You can put in a manual reset disc thermostat. (open on rising)
M/RESET Disk
That is in series with the safety contactor's coil.
This is the cleanest simplest way except it requires easy human access to reset. Works well in some situations and poorly in others. The safety contactor is just ON whenever power is applied to the system. You hear "clank" on power up.

2) You can put in a temp alarm who's output contacts are used to control the back-up (safety contactor). They use a temp sensor and are programmable.
One example
Another example

3) You can use a self resetting disk thermostat (open on rising) in series with a self latching relay.
Self resetting disk
You simply use the contactor's auxiliary NO contact in series with the disc thermostat. You parallel the auxiliary contact with a push button. When the system is started up you push the button. That pulls in the contactor which once pulled in stays pulled in as long as the systems is powered. Only the disk thermostat opening or a power loss drops out the contactor which then needs a human finger again.

You can fancy it up by using a Green LED panel light in parallel with the contactor coil to signify "OK". This can help with a human pushing a button and getting a pretty green confirmation that things are good.
OR
A Red LED panel light across available N.C. contactor aux contacts to signify alarm-tripped (no reheat). The feedback is more obscure in that the human finger makes the red light go out. Then it's more attention getting when it goes ON.

Of course you can use both lights and "have it all".

If the system cycles on and off frequently with no or little human intervention you can put in a one-shot relay to push-the-button once on start-up.





Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

itsmoked, thank you very much! I am have a weak spot for buttons with lights as well

 

[itsmoked]

You caught that eh?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

Keith is absolutely right, you need all sorts of direct acting safety devices.
By direct acting I mean not reliant on PLC, DCS or instrument power supply.

One that I remember was a lube oil heater controlled by the PLC with an RTD inside the tank.
Someone turned off the instrument power supply so the PLC thought to itself the oil is cold. By the morning we could have cooked fish and chips. From then on I always added a low temperature cutout in the logic as well.

 

[itsmoked]

Wow Roy that could've been reallly bad.

Azza; Back to the right way to do this next time.

The normal solution and one you could probably implement fairly easily on this job because there is already an established refer line route is this:

You add a smaller capacity condenser coil that fits the same exact duct physically like the evaporator. This coil is called a 'reheat coil'.
Because the reheat coil will have dry cool air blowing thru it, driven by the same blower running the air thru the evaporator coil, it can be a lower capacity condenser than the one outside, as it will be very efficient.

A tee is put into the hot gas line on the way from the compressor to the condenser. You add a refer solenoid valve to one side of the tee. From the refer valve you supply hot gas to the reheat coil.

Use a thermostat in the duct delivering the air or a room thermostat to run the solenoid valve. Air too cold valve opens. Air too hot valve closes. Using a highside pressure switch you control the original condenser's fan outside. When the reheat solenoid is open you will have the highside pressure dropping because the system has excessive condenser capacity. You maintain the pressure by simply stopping the outside condenser fan. No fan, it's like not even having a condenser outside. Occasionally the head pressure will rise high enough that the condenser fan will cycle on for a bit.

That's it. You save operational costs because you actually run the condenser fan less and you sure don't use resistive heating. This mod would probably pay back in a couple of months.

Keith Cress
kcress -

http://www.flaminsystems.com