Liquefier Main Compressor Cooling Summary of Musings as of 14-Feb-2022 -------------------------------------- This note is just a summary of my understanding of the cooling water flow for the main compressor in the helium liquefier. 1. The two plots from Cryo-Mech showing their minimum cooling water flow specification for the main compressor are on the web in the file: https://web.pa.msu.edu/people/edmunds/Helium_Liquefier/ filename: cooling_water_for_liquefier_compressor.pdf 2. Our Current Situation: Currently the cooling water flow rate is insufficient to allow the main compressor to operate normally. Without a window fan blowing air through the compressor it will trip Off with an over-temperature alarm. The current input water temperature is right about 23 deg C. and the output water temperature is about 42 deg C. giving typically a 19 deg C rise in temp. For 23 deg C input water temperature Cryo-Mech specifies a minimum flow rate of about 8.7 LPM. If I'm reading the flow rate gauge at the output of the booster pump correctly it says between 5 and 6 LPM or about 63% of the minimum specified flow rate. Last summer I saw the input cooling water temperature of 27 to 28 deg C on many days and in August I saw it as high as 29.7 deg C. For an input temperature of 27 deg C (the highest that the Cryo-Mech specification show) the require flow rate is a minimum of about 11.7 LPM. 3. Use the specified steady state power consumption and our Output minus Input water temperature difference to estimate what our current water flow rate must be: The Cryo-Mech specification says that the main compressor uses a steady state power of 9.9 k Watts. Currently our Output water temperature is typically 19 deg C higher than the input water temperature. 1 k Watt will cause a 14.34 LPM - deg C temperate rise. So with 9.9 k Watts and a 19 deg C temperature rise we must have a 7.5 LPM cooling water flow rate. For our current 23 deg C input water temperature Cryo-Mech specifies a minimum flow rate of 8.7 LPM. 7.5 LPM is about 86% of the minimum specified 8.7 LPM. 4. In the manual for the Liquefier look at the screen shots of the normal display screen and of logging screens to see what Input and Output water temperatures they had in their Cryo-Mech test setup and using the specified 9.9 k Watt stead state power consumption calculate the cooling water flow rate that must have existed in their test setup. Compare this calculated flow rate with the specified minimum required flow rate for the input water temperature shown in their displays. The Liquefier Manual shows: page 7-3 22.6 C Input 32.9 C Output delta 10.3 C page 7-5 22.0 C Input 32.0 C Output delta 10.0 C page 7-5 24.8 C Input 34.7 C Output delta 9.9 C page 7-10 22.8 C Input 32.8 C Output delta 10.0 C Call this a 23 C input water temperature and a temperature rise of 10 C - with 9.9 k Watts of heat added. So the flow rate in their test setup must have been 14.2 LPM. The minimum specified flow rate for 23 C input water is 8.7 LPM. Their test setup has a flow rate of 14.2 LPM which is 1.63 times the minimum required. So Cryo-Mech runs their test setup with plenty of cooling as would be expected for a serious and expensive piece of industrial equipment. To help us achieve a long system life we should use plenty of cooling water rather than just hanging on by a thread. 5. From an engineering point of view it would make sense that they want to limit the temperature of the hottest point in their system, i.e. independent of the input water temperature, the output water temperature must be no hotter than bla. Can we study their graph of the required flow rate vs input water temperature to revers engineer what the temperature bla must be ? Look at low, middle, and high input water temperatures: 10 C input temp requires a flow of 4.54 LPM 4.54 LPM plus 9.9 KW of heat --> out temp = 41.3 C 19 C input temp requires a flow of 7.15 LPM 7.15 LPM plus 9.9 KW of heat --> out temp = 38.9 C 27 C input temp requires a flow of 11.7 LPM 11.7 LPM plus 9.9 KW of heat --> out temp = 39.1 C So perhaps they are trying to keep the outlet water temperature below about 40 C. The outlet water temp in the Cryo-Mech test setup was always below 35 C. We are currently running with 42 C outlet water. 6. How constricted is the flow of cooling water through the compressor ? We can study the Cryo-Mech graph that specifies: pressure drop vs flow rate to see if the flow rate is proportional to the pressure drop or is the flow rate proportional to the square root of the pressure drop. The second case is the normal situation where the required force goes as velocity squared. Look at low, middle, and high points in the graph of flow rate vs pressure drop. Flow Specified Flow Rate Rate Pressure Drop Square Root Divided by LPM psi Pressure Drop SQRT Press Drop ----- ------------- ------------- --------------- 5 5.1 2.26 2.21 8 12.0 3.46 2.31 11 20.9 4.57 2.40 So it's pretty clear that the cooling circuit in the compressor is constrained enough so that it requires about 4 time the pressure to double the flow rate. An interesting point at the very low end of the Cryo-Mech graph of flow rate vs pressure drop is that perhaps you can just see the start of a linear relationship. 7. The highest inlet water temperature that we recorded last summer was 29.7 deg C (call it 30 C). The highest inlet water temperature that is covered by the Cryo-Mech graphs is 27 deg C. But based on what we learned in points 5 and 6 above we can extrapolate to find the required minimum flow rate for 30 deg C inlet water and the resulting pressure drop at that flow rate. - With 30 deg C inlet water we need a flow rate of 14.2 LPM to keep an outlet water temperature of no more than 40 deg C. - With a flow rate of 14.2 LPM we should expect a pressure drop of 35 psi going through the compressor water circuit. These required flow rate and pressure drop numbers would help in selecting a booster pump large enough to allow normal operation of the liquefier with the warmest known summer process water temperature. 8. For completeness let's not forget the rest of the Cryo-Mech cooling water specification: Maximum Inlet Pressure 110 psig Alkalinity 5.8 < pH < 8.0 Calcium Carbonate Concentration < 80 ppm