In a water cooled injection molding machine one closes down the output valve on the coolant line to increase the thermal transfer and speed mold cycling.
In a counterflow chiller the amount of heat transfer depends on many things, among them the ratio of the coolant flow to wort flow rates. If you increase the coolant flow rate and/or decrease the wort flow rate the wort will exit at a temperature closer to the coolant inlet temperature. And conversely. But the relationship between "efficiency" (defined as the drop in temperature divided by the initial difference in wort and coolant temperature) is not a linear function of the ratio. A lot has to do with whether coolant and wort flow are laminar. If either flow is increased to the point where the flow is no longer laminar transfer will be effected. And even where the flow is laminar the relationship between ratio and efficiency isn't linear. Each chiller can be characterized by a parameter I call Q, which depends on the geometry of the chiller. It's units are gallons per minute (or liters per second or whatever you will). A good operating point for a chiller is where the coolant flow rate is Q and the wort flow rate Q/5. This results in 99% efficiency (e.g. wort inlet temp 200, coolant temp 50, wort outlet temp 51.5 Â°F) with 5 times as much water used as wort is cooled. Increasing the coolant flow beyond this point has little benefit. Reducing it to 0.2*Q results in a decrease in efficiency to about 95% (exiting wort temp 56.5 Â°F) in the example.
Also slow the flow Water is a phase change medium...
There is no phase change in a properly operated wort chiller.
...and absorbs thermal energy differently at different temperatures.
No it doesn't (at least not appreciably - the specific heat of water changes by less than 1 % over the entire range from freezing to boiling).
Easiest way to see this is to pop a thermometer in a pot and watch the thermometer and a clock plotting the time/Temp on a graph while the water comes to a boil. You'll see that during the last several minutes before a boil the water temp climbs painfully slowly.
That's not because of a change in specific heat but rather because the rate of heat transfer between two systems depends on the temperature difference between them. Thus as the water gets hotter less heat gets transferred to the water from the heat source while at the same time more gets transferred from the water to the surrounding air through the walls of the vessel. Also as high temperature the vapor pressure of water is approaching 1 atm and even though not yet in full ebulition, a lot of heat is being carried off in that vapor: 2270 Joules*/g. Note that this is over 4 times the energy which went into that gram to raise its temperature from just above freezing to 99 Â°C. Clearly, loss of latent heat through vaporization would be the major reason for the decreased temperature rate in most geometries.
Inside a wort chiller where there is no vapor (unless in turbulent flow and no place for the vapor to go if it is so it can't carry away heat) the temperature of the coolant will change 1 Â°C for every 4.219 Joules*/L put into it at near boiling and 1 Â°C for every 4.181 J/L put into it at 20 Â°C.
*Joule (after whom the unit of energy is named) was the son of a wealthy brewer and did much of his seminal work in a laboratory attached to his father's brewery).