What do you suggest to control air conditioning units? I have more than 10 over several distances (farest unit at 120 mts) I need (of course) to keep all in lowest cost and fully functional. Just give me a clue and I will dig all out. Thanks in advance

wireless sensors for humidity and temperature, rtu with modem, scada server to set parameters and monitor operations, plc if necessary, servo/stepper controlled valves for air regulation

That's too limited info, is it inverter compressor or fixed speed, TxV or ExV, condenser fan is it variable speed or fix speed.

There are several routes you can take however it does depend on physical distance between units to how cost effective you can make the solution. I would suggest a remote communications option although the hardware cost will be more expensive it will take a less time to install therefore less man hours. In Europe, this is an issue, as labour cost far outstrip the hardware costs this may or may not be a factor. Other than that any of the PLC manufacturers have the equipment to achieve this in a traditional wired method remote I/O method. More info would be requires to assess which way would be suitable:

1. I/O points to be monitored on each unit: i.e. Stop, Start, Fault, Running etc...
2. Are the fans to be independently controlled on each unit.
3. What sort of visualisation is required?
4. Distances between units (this allows us to assess whether two units can be grouped cutting down on cost when installing a remote communications system).
5. Temperature measurement: What sensor? Do you plan to use a transmitter?
6. Modulating valves do you have them? If so 0-10v 4-20ma?

Networks for consideration:
ASi: (Actuator Sensor Interface) Standard distance 100m although easily expandable to 200m and can reach distances of 600m

Profibus/net: Quite expensive solutions but will stand the test of time

Device/control Net: Quite expensive solutions but will stand the test of time

CAN Bus (or similar): I don't have as much experience of these as I would like so I cannot comment on their reliability.

Biggest question you have to ask is where is your cut off point between labour cost and hardware cost and is usually one of the hardest questions to answer as well.

Integrate all units in a low cost plc with a incorporate Tft display ( to use like scada). I suggest a crouzet that i use in various application and until now is very running with no problem and the cost is very low.

What kind of units are these, central air conditioning, window AC , or split AC. What all things u want to monitor and control. 120 mts is not considered as along distance and how cost effective you need it. There are a lot of solutions.

You may please try out a Direct PC based system like from Beckhoff etc.

A Very cheap possibility is using a Raspberry PI with a webserver, which has already digital I/O on board, and is costing only about 40$. I use it myself to control heating in a building by my smartphone or tablet or PC. The little unit can have a webserver on board, so I just surf to the webpage and set the temperature. By the time I arrive in the building it is nicely warmed up. If you Google Raspberry I/O or something you will see how easy it is.
A more industrial (but also much more expensive!) solution is to us eWon Routers. Also on this you find plenty of information on their website.
Both solutions work over a network, so I am assuming there is one available (ethernet or Wifi).

For such limited I/O applications, ethernet is apt since the end customer does not favor wifi.

Dear Jose.

Before you go jumping into the thermodynamics of a refrigeration condensing unit ie tampering with high and low side parts of a refrigeration system where you might actually cause damage ie "slug the compressor" or simply over heat it by trying to control the condensing side of the system. Typically or at least on refrigeration systems, there's already components to maintain even pressures through out the "phase changes of a system, head masters, fan cycling based on ambient temperatures etc. I am still trying to figure out what you are trying to accomplish here? perhaps you might repost to all of us what you are trying to do and give us some more specifics. Whats the tonnage of these units, 5, 10, 10, 20, tonnes? Whats the application, a business facility controlling room temperature, is this part of a chiller system? VAV valves anywhere in the ducting? As far as controls, ip network over cat6 to a scada server out to plc(s) on a ip address is simple enough. If your trying to save on energy it is imperative and the most common cause of air conditioning problems is dirty, plugged condensers full of dirt, dust and an just build up that will drive the head pressure up astronomically especially R22 based systems...need more info....cheers