Heat pump - Pressure and temperature in the condenser.

[arne22]

Hello !

I/we are doing a inverter control system for a full scale laboratory heat pump that shall/will be inverter controlled. From asking a few qustion in this forum before, I got some information and hints to find even more information.

The advice was among other to search and to find some info about existing commersial heat pumps. So I did, and the information on the Daikin heatpump was quite good and informative. Link

What I have found out is that Daikin uses a PI regulator + an inverter to set the speed of the comressor and that there is also some other means of setting the spead of the evaporater/condenser as well. (But more using mure simle solutions like using steps like low-medium-high rateher than PI or PID control.)

The Daikin dokumentation referes to the condenser temperature as the feed-back signal for the comressor speed controller.

In some way this sounds reasonable as what comes out of the condenser is heat.

On the other hand what the compressor makes is not (in the first place) heat, but rather "pressure", and then as a consequense of that, it comes also temperature. If you messure the temperature at the outher surface there should (teroretically) be a time delay between the change in pressure and change in temperature, at least if measured at the outside of the condenser. (If it is measured at the outside.)

One thing I wonder about: Why don't they rather measure the pressure inside the condenser and use this signal as the feedback to to the compressor speed loop ?
(Should't this give a shorter time constant and a more presice compressor control ?)

One other "hidden" or related question: What is the standard relation between temerature and pressure in a condenser, using a standard referigation media ? (R135A or other)

If temperature goes up "a little" lets say from 45 to 50 degres Celsius (if this is reasonable values), what will then happen with temperature, will it rise a lot more or a lot less, or is it possible to say anything about that ?

Could it be some certain reason why Daikin uses condenser temperature and not condenser pressure as the feed-back to the compressor control loop ?

Anyone with some interesting ideas about this ?

Excuse me for posting "to much" about the same subject, but it actually helps, and I think we are now rather close to find out how to do it. Feed back from this forum and the Daikin dokumentation actually helped quite well.

Best reg Arne.

[sedgy]

hi arne22,
I was in the trade , but have now retiered, and I found your thread very interesting cos I have A PANASONIC invertor and found that to moniter the unit , I had to moniter the the air temp in and out with two cheap thermometers< digital < I would like to know at what conditions the unit is running at its optimal output ?< this is an open question to all readers < sedgy,

[arne22]

Hello sedgy ! (..and everybody)

i'm almost a newcomer in the "area of heat pump technology" but I have worked with automation systems for a couple of years. (Different types of industrial control systems.)

I think that to calculate the overall factor for how well a heat pump work, you will have to measure the delta T (in the airstream) for the evaporator and the condenser, and you will also have to know the airflow trough both uints. This will be rather difficult to measure with some precision, but some pluss minus 10 percent or something like that, should be possible. (I hope)

Then there will also be necessary to know the electrical power of the engine/compressor, that should be a little bit more easy. (Event though that the inverter will make a shape out of the electric power that will make it difficult to measure.)

From the parts we have collected to the project there is only one inverter for the compressor. The evaporator is also divided into two section each with a magnet valve, so we should be able to run with one or two "evaporator sections". I realize now that it should/might also be a good idea, if we were able to tun the condenser and the evaporator fans also with inverters, to be able to vary the airflow as wanted. (This should also increase or lower the capacity of these items, so these factors can be controllable.)

When it comes to control systems using inverters, the factory manuals, (at least not those I have found until now) does not explain to much how things really works, it's more "how to connect cables".

But from the "connection of the cables" I think it is possible to make that overall conclusion, that If you want to do a compressor speed control system, using an inverter, this will normally not be a "ordinary single control loop", it will rather be a double loop, or a "cascade control loop".

In this cascade control loop system, the inner process or control loop will typically be the compressor speed control, that receives its feed back from the condenser temperature (Daikin documentation), or possible some other feed back signals from the "internal processes" in the heat pump. Then there is this other "long time constant process" that is the room temperature process itself. This is the "outer control loop" in this control loop system. The heat pump manuals does not directly say it works like this, but it looks like it does.

By the way, we are using/will be using a Copeland scroll pump, and I don't know how capacity/pressure will be a function of speed for this pump. "Rumours" have said that this pump only con run with speed from 45 Hz to 60 Hz, and that it will be damaged over and under this speed. This is not much of a control area (?) for a inverter, but possibly, if the condenser fan speed also were "trottled" up and down, this might give something together, as a result of both factors.

Any information that could put some more information of how these things works would be very welcome.

Best reg Arne.