We want to modify some test equipment we have. Currently, that piece of equipment basically heats up the liquid in it to a certain temperature (based on user input in computer program) and then stops. It kicks back in once the liquid's temperature reaches the lower limit. Our goal is to somehow make the cooling process faster by adding some sort of cooling coil and fan next to the unit. Once the high limit is reached, we want the liquid to circulate through this cooling coil and have the fan turn on.

Basically we want a temperature sensor to send a signal to the unit and have the unit turn on the fan and open the 3-way solenoid valve (to circulate the liquid through the coil). As long as the temperature of the liquid remains between the upper and lower limit, the fan will stay on and the solenoid valve open. We don't care about controlling the fan's speed. Even if the fan runs at maximum speed during the entire time that the lower limit is reached, we are fine with that. Once the liquid cools down, we want the solenoid valve to close and shut off circulation and the fan to turn off too.

One last thing, we would like for this unit to accomodate liquid temperatures of upto 300 degrees F. Let me know if you have any recommendations or suggestions. Thanks in advance.

 

Any on-off controller with a Form C relay output and adjustable deadband or hysteresis should do it. The one on my bench here is a Honeywell UDC 2500 (Model DC2500-Ex-).

The hysteresis is adjustable and control output relay is Form C with Common, N.O. & N.C. contacts.

Using the UDC 2500 as an example:

Connect load power to the common on terminal 20.
Connect the heater load driver (presumably a contactor) to N.O. terminal 21.
Connect the cooling load (presumable a contactor to drive the valve, fan, whatever) to N.C. terminal 19.

The connection from common 20 to terminal 21 will be made during the heat-on cycle, supplying power to the heater.

The connection from the common 20 to terminal 19 will be made during the heat-off, cool-down cycle, supplying power to the cooling valve.

Settings particular to this configuration:

ALGOR > CTRALG = ONOF
OUTALG = RLY
CONTRL > ACTION = REV
CONTRL > HYST = XX.X

HYST = output hysteresis: the gap between the heater turn-off point and the heater turn-on point in percentage of span of the input sensor (not in units of degrees).

For example, for a TYPE J thermocouple with range spanning -7 to 288°C, the span is 281°C (288 + 7 = 295)

A heating target SP (turn-off SP) of 160°C, with a turn-on SP of 100°C, would use a 60° hysteresis value: (160 - 100 = 60)

60° is 20.4% of the 295° input span.
So the HYST setting = 20.4

A univeral output model DC2500-CE does the same thing using the relay on terminalS 4 (N.C), 5 common & 6 (N.O.)

Bud

 

Whoops, my mistake. I forgot Honeywell splits the hysteresis band...

Those controllers do not do straight on-off with the off point at setpoint in reverse action.

Honeywell splits the hysteresis band around the setpoint: half of the hysteresis band above the setpoint, half of the setpoint below the setpoint.

Probably so that a typical heating operation places midpoint in the middle of the on-off heating oscilllations.

But, that 'split hysteresis' means that to accomplish the example, the setpoint is not 160°C, but 130°C.

With a setpoint of 130°C, the heater stays on up to 160°C, turns off at 160°C, cooling connection is made to the other contact until temperature drops to 100°C, where the heater turns back on again.

It's not intuitive for me to have the setpoint not at one of the switch points, but that's how that little box works.

Bud

 

You already have all the control electronics you need.

The existing system turns the heater on and off (in a fully controlled way).

All you need to do to achieve your goal is have the fan and solenoid valve do the opposite of what the heater is doing. In other words just wire the fan and valve in parallel (I'm assuming they were purchased to run together, so they would need to have the same voltage requirements), and power them up at all times the heater is not "on". One relay is all you should need to add.