Engine design and high temperature cooling systems
There are a few engines available in the market today that are designed for high temperature cooling systems. These cooling systems typically operate at temperatures of 127o C with the recovered jacket water heat used specifically for the purpose of generating steam. Higher temperature jacket water cooling systems of typically 110o C are also used in landfill or biogas applications where there are impurities in the gas and hence need to maintain elevated engine temperatures to hold the impurities in suspension in a gaseous state to keep them from condensing in the engine. These temperatures are usually higher than the boiling point of water under normal atmospheric pressures. These temperatures are only on the jacket water side on the cooling system.
Keeping the water in the engine in a liquid form at high temperatures
Another important issue to contend with is hot shutdown conditions. During this condition there is a tendency for the water near hot areas like turbo chargers to steam up and thus high temperatures are reached, as in steam form it can become super heated. It is important to maintain the cooling water at a pressure constantly above the boiling point even with the engine stopped.
This poses a real challenge for conventional cooling systems. The hot shutdowns and subsequent steaming has led to failures of turbo chargers and also led to corrosion in the cooling circuits. The conventional cooling system with expansion tank and pressure cap is supposedly pressurized too, but when the engine stops and cools the pressure turns down. Also the air in the expansion tank will cool faster than the water and can release the pressure faster. Hence there is a need for a better pressure regulating system that maintains cooling medium pressure at all times.
There are many systems available in the market to do this function, but the most economical and commonly used is a bladder expansion tank system. The bladder expansion tank is an accumulator vessel with a bladder diaphragm inside. These expansion tanks provide a pneumatic cushion capable of continued operation without the loss of air into the system through absorption. This is possible because a flexible diaphragm (bladder) separates the water and air in the tank. The permanently sealed-in air cushion can be pre-charged to minimum system pressure to permit the smallest tank possible. This pressure balancing is what provides the required pressure of the water all the time.
Let us now examine the pressures needed for the various water temperatures. If water were to remain liquid at 101 deg C then a 7 psig pressure is needed in the system all the time. For a 110 deg C operation we need to charge to 14 psig. How do we achieve this in our cooling system design?
A typical diagram is given below:
- Here, the air is prefilled in the bladder tank to the required balancing pressure. In the case of 110 Deg C to 14 psig or 1 bar.
- Water is then filled into the cooling system to from the bottom most point using a pump and the system is fully bled of all air.
- The vent valve is then closed and the system is pressurized to 14 psig and the filling valve then closed.
This first provided the balanced cooling system. The sizing of the expansion tank is done so that when the water expands to the volume at 110 deg C the pressure in the system added does not exceed the maximum operating pressure of the system. The operating pressure should be such that the increase does not exceed 50% of the initial charge, i.e, if the initial charge pressure is 1bar the final operating pressures not to exceed 1.5 bar. Once this system is balanced the presence of positive pressure at all time keeps the system from steaming and any flashing will quickly dissolve back into the system. Another addition to the system can be a de-aeration chamber that will separate all the micro bubbles that are formed due to the pump churning action. This is usually located at the topmost point.
From the short discussion above you can appreciate that a properly engineered high temperature cooling system is paramount for a successful operating system. Care taken during the system design stage and installation will result in long engine life providing the owner / operator the expected return on investment.
- Have you ever used bladder expansion tank system? If so, what was your experience?
- Are there other considerations not mentioned above?
- Do you have experience with other types of systems? Please elaborate.
- Have you faced any problems in filling and pressurizing the system from your experience?
- How do we size the bladder expansion tank and the initial charge pressure?
- How do we prevent excess pressures from developing in the system?
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