Gentlemen,
First of all, I would like to thank everybody has participated in this forum. I have been involved for the last year or so and have learned a great deal about HVAC from this forum. You can only get so much from books.
I kind of work on my own and am self taught with a lot of things, so I would like to throw this question out to the group.
When faced with a simple control system to be designed/built I am often faced with making the decision between using a small inexpensive PLC or just using a number of relays.
For example, I build small (up to 30 ton) process water chillers, many for the medical market. MRI and CT Scanning. We build a city water switchover device, that will shut down the chiller and switch some solenoids to run city water thorugh the MR and down the drain for emergency cooling. A couple of time I have had engineers spec to have dual pumps in the chiller and the city water switch over so I had to design logic for when to switch over to city water. Without going into design details, what I decided to do was use (2) alternating relays, and a programmable relay, one of those ones with a timer setting and a function setting. It was set to pulse the relay 5 sec on and off when the input contact was closed.
THe cost of these parts was over $200.00. I could have done the same thing with simpler wiring for 120 using a er, small fixed IO plc. However, I shied away from the PLC (I am an experienced user and programmer) because it has an "air" of complexity that would freak out some people. Even though it makes the wirng a million times easier and the whole thing cheaper, it seems that when people see this little black box they tend to look suspiciously at the equipemnt. This is epsecially true (at least I have seen) in the HVAC industry. If it does not say Trane, York, Johnson Controls, Honeywell, etc, then it must be junk and/or from Mars.
In many ways, a more sophisticated controller can make a job more reliable, and often cheaper. But it can come at the expense of usability.
My question then is where do you draw the line between complexity and usabity? What criteria would you use on deciding on a control scheme for something as simple as I described above? Do you agree that there is a perception problem with controls?
Everyone wants something that is simple, but powerful, with an ifinite lifespan that will cost $2.79.
Thanks for you input.
Clyde
First of all, I would like to thank everybody has participated in this forum. I have been involved for the last year or so and have learned a great deal about HVAC from this forum. You can only get so much from books.
I kind of work on my own and am self taught with a lot of things, so I would like to throw this question out to the group.
When faced with a simple control system to be designed/built I am often faced with making the decision between using a small inexpensive PLC or just using a number of relays.
For example, I build small (up to 30 ton) process water chillers, many for the medical market. MRI and CT Scanning. We build a city water switchover device, that will shut down the chiller and switch some solenoids to run city water thorugh the MR and down the drain for emergency cooling. A couple of time I have had engineers spec to have dual pumps in the chiller and the city water switch over so I had to design logic for when to switch over to city water. Without going into design details, what I decided to do was use (2) alternating relays, and a programmable relay, one of those ones with a timer setting and a function setting. It was set to pulse the relay 5 sec on and off when the input contact was closed.
THe cost of these parts was over $200.00. I could have done the same thing with simpler wiring for 120 using a er, small fixed IO plc. However, I shied away from the PLC (I am an experienced user and programmer) because it has an "air" of complexity that would freak out some people. Even though it makes the wirng a million times easier and the whole thing cheaper, it seems that when people see this little black box they tend to look suspiciously at the equipemnt. This is epsecially true (at least I have seen) in the HVAC industry. If it does not say Trane, York, Johnson Controls, Honeywell, etc, then it must be junk and/or from Mars.
In many ways, a more sophisticated controller can make a job more reliable, and often cheaper. But it can come at the expense of usability.
My question then is where do you draw the line between complexity and usabity? What criteria would you use on deciding on a control scheme for something as simple as I described above? Do you agree that there is a perception problem with controls?
Everyone wants something that is simple, but powerful, with an ifinite lifespan that will cost $2.79.
Thanks for you input.
Clyde
