Economiser/subcooler's

[malcopate]

Hi Guys, working on a new proto-type chiller would like a bit of advice on the use of the above on the said chiller.At present the LLS Valve supplying the Eco is always energised in all ambients from -10 to + 40 0c It is my feeling that this should only work on eco mode between ambients around +15 to +25 0c any advice would be appreciated.

[The Viking]

4 1/2 ???

Malcopate,

Any chance you could provide a bit more information?
At the moment your post is like asking what pressure a tyre should be inflated to but without even telling us what tyre it is we are talking about, is it for a toy bicycle or a 120 tonne digger....

What are the design criteria?
What do you want to achieve?
What is it feeding?
And So On.



.

[malcopate]

chiller operating parameters -15 to + 40 ambient,r134a,800kw cooling capacity,will be in all types of applications from hi ambients to very low ambients,at present the eco is in operation all of the time,running with bitzer screws(160 and 210) looking at the design envolope i can see that the capacity will reduce if it is still got an energised sol valve,not sure how to calculate when it should not be on so to be as low ambients it will get over condensed and at high ambients it will lose some of its liquid to subcooling its own liquid hence capacity reduction pls help if u can m8 any other info u want let me know

[sterl]

The basis of the economiser arrangement is that the vapor generated by subcooling the bulk flow of the liquid is being compressed from a higher suction pressure than that devoted to compressing the vapor from the evaporator. Obviously both suction conditions are driving toward the same discharge condition. To understand when to do it or not to do it, you need a performance curve for the compressor's main suction at the conditions of operation and a performance curve for the side port suction condition at a variety of discharge pressures; and depending on how you are doing capacity control: you may need the performance curve for the side port at a variety of slide valve positions. Some of the Bitzers are equipped for both open port (bypass partially compressed gas back to suction) and slide valve control. With slide valves: the actual pressure of the side port is decreasing toward the main suction as the machine unloads, so the vapor associated with the economiser is going through the same compression ration as that of the main stream, only the regulator maintaining the econmiser vapor pressure is operating at a higher pressure difference so the cooling effect did not change....Only the power input. The 2-curves cross each other at some point of slide valve travel given a specific volume ratio of operation. With variable Vi, both SV position and actual operating Vi become determinants. The manufacturer can estimate when that crossover takes place but its pretty typical that at 80% SV position and high condensing, the efficiency gain disappears and you are actually more efficient to simply take the flash gas at the evaporator. At low condensing the economiser is operating at a low load anyway and the side port is closer to machine suction so the typical crossover is higher in terms of the slide valve position...Though at low condensing the reduced mass flow of the econmiser makes the actual cut-off less of a concern.

To make a decent stab at all this: You will need to know the positions of side port, slide valve travel and Vi stop range of effectiveness and then base everything on a polytropic compression model to develop the pressure of the side port at a variety of operational conditions.

You also need to apply to all that the role of the controls and what represents a tolerable liquid temperature. For a chiller, you may be able to let the subcooled liquid temperature float down and operate with a regulator set real low. This expands the range of applicability; but your heat exchanger and expansion device may both suffer if all the flash gas is knocked out of the process by low liquid temperatures.