I am designing a shell and tube heat exchanger to cool the lubricating oil of an air screw compressor to replace in the existing air-cooler.
The oil is Iso VG-68 and its flow rate and operating pressure are determined. However, the inlet and outlet temperature of the cooler are unknown in design case. The only specified temperature is the maximum allowable temperature of the oil in which the compressors' motor will be stopped and this temp. is 85 C.
The vendor's documents are incomplete and the vendor itself is inaccessible to reply which inlet and outlet temps. to be considered.
I would appreciate if someone has a reliable document or recommendation to select the best operational temperatures for designing an appropriate lube oil cooler such as my case.
Your question is a valid and practical one. Your problem is complicated by having to work with an unreliable and uncooperative screw compressor manufacturer and supplier.
Today, there are two types of screw compressors being employed: flooded and dry. You fail to tell us which one you have, so it is hard to get an idea of what the temperature of the recirculated oil (before the cooler, really is - or should be. My experience with screw compressors has been primarily with the flooded variety and reputable manufacturers like Mycom and Atlas-Copco. They always responded to spare parts, operating information, and service requests.
If you can’t obtain any information from the manufacturer and you have no operating manual that details out the operating temperatures and flow rates of your oil system, then you have a serious problem with your supplier and the best thing would be to cease doing any further business with them. It appears you are left “out on a limb”, without any backup from your manufacturer. You need more than the oil type and cooler inlet and outlet temperatures in order to design an equivalent water cooled cooler. Primarily you need the oil design flowrate. If you don’t know that, you can’t get started on any cooler design.
Why replace the air-cooled model with a water-cooled one? If you can’t obtain a water-cooled model from your manufacturer, then I would just continue with the air-cooled one.
I can tell you from field experience that your cooler outlet oil temperature should be 100 to 120 oF - depending on your oil’s viscosity ratings. But I can’t even guess what the design flow rate should be. The manufacturer is the one who determines that.
As I mentioned in the first post, the flow rate can be specified from the oil pump data. And, I have considered the outlet temp. 50 C ( about 120 F). However, the important issue is the inlet temperature of the recirculated oil. If it is specified higher than the expected, I will have a large cooler and I waste the money for the over design case. On the other hand, if it is taken lower than the expected, the cooler cannot cool the oil enough and its destiny will be similar to the existing air-cooled exchanger in which the outlet temperature of the oil is high specially in summer. The company is RKR and it is in our blacklist.
Please let me know what inlet temperature shall be commonly considered in such cases.
Edited by Mahdi1980, 22 May 2017 - 05:27 AM.
I don't think the cost saving relevant to a lower Tin (duty) could be very high (but the size of your plant is unknown to us....).
Better considerations could be done by analyzing the documentation of the package and/or plant data.
Is there any other comment regarding the range of the inlet temperature of the lube oil into oil cooler?
You should be able to estimate the inlet temperature by setting the outlet temperature, as you have suggested, then rate the existing air cooler for varying inlet temperatures. When you get a match for duty (you know the flow) for the available air cooler surface, then you can reasonably expect to have a good estimate of the original design inlet temperature. Then add whatever safety factor you think is appropriate.
However, I do not have any information about the existing air cooler such as the air flow rate and the surface area of the heat exchanger.
Moreover, it is the fault of designer regarding this cooler and if I had enough information, the data would be unreliable.
Due to the above issues, I am looking for the optimum inlet temperature to the cooler.
the CW supply temperature is 35 C in design case.
I have considered the inlet tem. of the oil 65 C and the outlet 50 C and I am designing the cooler with 10% excess area.
If you and other fiends have any comment please let me know as soon as possible.
But, as a process engineer, I would stay a little bit more conservative with respect to your assumptions.
How many $$$ do you think to save by specifying 65 instead of 75°C?
For your information, I designed a shell and tube heat exchanger as an oil cooler based on the oil inlet temp. of 70 C and outlet temp of 50 C and 5% excess area.
I hope it works well in summer after its construction and installation and be a good replacement of the existing air cooler of the lube oil.
However this is going to increase your cost..but is best option in case of uncertainty.
We have the similar design for our offshore Gas Lift Compressor lube oil cooler.
Just read your post, please correct me if im wrong, but if your screw compressor is a flooded one then the lube oil cooler inlet temperature should be the same as the discharge temperature of process gas (air in your case) at compressor discharge. The reason for my understanding is as ive seen in refrigeration oil flooded compressors, the temperature of both the oil and refrigerant is the same in the KO vessel where they are separated and the oil goes to its path thru filter and cooler to be then reinjected into the screw.