Small 12 vdc Motor/Pump Assembly

Have a look at Grundfos Pumps Australia - they may have something - I use one of their 3 speed pumps.

You could look into what the beverage industry uses for soda fountains. The pumps that mix the drink concentrate with the carbonated water can pump at a low rate, and can change speeds if you get the right one. But even then, those might pump more than 1 liter per hour. That is very very slow.

Your quite slow pump rate could be provided by placing the fluid in a raised tank and use gravity feed to supply the required flow. That way you could run two seperate lines from the tank. Seperating the two output feeds from the tank physically (distance) should minimise any effect that one line has on the other. What is the medium anyway?

medium is 44 deg water used in a tube in tube heat exchanger. Water is fed through the outer tube, and the colder fluid is fed through the inner tube which is simply fed by gravity. The colder fluid head pressure is no more than a couple of feet of H2O. The tube itself is a soft plastic.

44 deg C I meant to say, and the apparatus must be compact which would not allow for an elevated reservior.

Fluid Metering, Inc. makes very small pumps with encoders built in to calculate flow rate and probably has what you require. A smart controller may be required. http://www.fmipump.com/

Can you consider a dual-head peristaltic pump? Flow rates would be automatically synched, and either one set of rollers could be unloaded (like is done to change tubing), or the valving might be switched to two-way diverters (routing the unused side back to its source but permitting the pump & circulation to be uninterrupted). Does the switchover have to be automatic? Under electrical (pneumatic, hydraulic, or whatever) control?

Actually, that would work wonderfully. Perhaps you are aware of a source for such a pump. It would have to be electrically controlled. The working fluid is re-circulated during the whole process as it is constantly being drawn from a reservior and then dumped back in once the the heat has been removed by the colder fluid. Ideally, the switch over would be done at the touch of a button, or with minimal user intervention.

Rather than a peristaltic pump, a gear pump will give pulseless flow. Cole-Parmer offer a wide range.

Those are valid, but expensive alternatives.

Any cheap 12VDC pump/motor combo should work if it has a flow rate in the range you need. Do not oversize the pump though. 12 VDC motors are easily variable by applying reduced voltage through using a variable resistor or "pot" (potentiometer) in series with the motor. Your power requirements must be very low for such a low flow rate, so the "pots" could be very small as well (resistors must be large enough to dissipate the power you are "dumping").

Simply wire in a toggle switch to the positive voltage (battery or DC power supply), one way for HI and the other way for LO, or even a 3-way switch for HI, LO, and OFF. Connect a "pot" to each of the two switch outputs, then wire the other side of the pots together and connect them to the motor (positive). Connect your voltage source to chassis ground (and ground the other motor lead as well) or directly to the other motor lead.

Check your flow rates and adjust the pots as necessary. Then either lock down the pots in position or mark the setting and guard against adjustments (e.g. inside a locked box!)