DDC or PLC ?

Y

Thread Starter

YO

I know the prices of DDC are mostly lower than PLC, but when installing a new HVAC systems, with 10, 20 or even 30 Air Handling Units (AHU), one PLC that controlls all AHUs would be cheeper, isn't it?
All AHUs are simillar.

Does PLCs now have ready control modules for processes like these? which Vendor?

I need your opinion(s) regarding this issue with aspects of robustness, programming, working life time, networking, PID setting and more.

Thanx
YO
 
P

Paul Van Aeken

Working for a company doing maintenance of technical installations HVAC. Working with DCC and PLC, after 1992 switching from DCC to PLC(Siemens step5-step7). with 1 Step7-300 PLC we can command at least 10 HVAC units, networking is easy,(profibus+ethernet), scada as well (citect and winCC) and no need to pay crazy software upgrades. (DCC scada upgrade ==extreme expensive).
maintain 300 buildings (24/24)with a dispatching and no problem.
Regards Paul
sorry for my poor english!
 
Yes, at the end of the day the PLC will probably be cheaper. With DDC systems typically the DDC people charge very unrealistic prices for variations etc.
Most PLCs have PID functions etc built in these days. The latest CS1H and CJ1 PLCs from Omron have a 2 degree of freedom auto tune PID function built in. Turn on a bit, the function tunes itself and turns off the bit. Could not be easier.
 
Hello YO,

Our 20 year experience is that DDC is the way to go for HVAC control. These controllers are built for the application. The better ones have inputs ready for low cost thermistor or PT temperature sensors; built-in functions for calculating enthalpy, dew-point temperature, etc.; algorithms for optional start programs, chiller sequencing, etc., and much more.

You mention tens of similar AHUs. I assume these are spread around a building or buildings. How would you connect one PLC to all of them? Hard wiring? How much would that cost? I/O modules? What about reliabilty?

A small DDC controller for an AHU will cost you under $300 including the application programming. Install it in or near the AHU's motor control panel. All the I/Os are within the box or a few meters away. Minimum installation cost and no probems of wiring errors, maintenance, etc. Every controller functions stand-alone, communication between controllers is peer-to-peer. An extremely reliable, fully distributed architecture that's easy to understand, install, and mainatin.

You've got to be an incurable PLC addict to do otherwise.

Good luck,

Zvi
 
Zvi,

Installing one small DDC to a single HVAC unit seems to be the easiest way as you describe it.

The limited number of I/O per DDC may lead to install larger (and more expensive) DDC when coming to a high performance HVAC systems (for pharmaceutical industry).

Add to this situation the requirement for HMI and data aquisition (for regulatory purposes)and you easy come to a situation where - for a large quantity production plant - you must communicate with more tha 100 DDC. This is a heavy burden on the HMI communications.

Do you have a solution for that?

Thanx
YO
 
YO,

The DDC controller I had in mind has 8 I/O. A 16 I/O controller would cost about $500 and a unit with 28 I/O under $900.

We have a number of applications with 100 DDC controllers and more. There's no HMI burden since the controllers function peer-to-peer. Scheduling and data acquisition tasks are all performed by the controllers, independent of the HMI.

The computer HMI is only needed for archiving and operator interaction. Most of the time it can be turned off.

Zvi
 
A little deeper into the site you will find S7s.
http://www.landisstaefa.com/sys/e/sys_sic.asp Take a look at the automation
level. This site is from the European side and the Siclimat is mainly sold in Germany.

I'm biased. If you want an HVAC Control System, buy one (fm SBT). Don't waste time designing, installing, configuring and maintaining something that is not central to your business.

Best,
B.O. Apr. 29, 2003
--
Robert Old, System Architecture, [email protected]
Siemens Building Technologies, Inc., Building Automation
1000 Deerfield Pkwy., Buffalo Grove, IL 60089-4513 USA
Phone: +1(847)941-5623, Fax: +1(866)699-0126
 
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