Tube and Shell Coolers
The tube and shell is the most common type of heat exchanger. They are used to remove heat from compressed air and gas. They are also used to remove heat from oil, when oil is used as a cooling fluid in a piece of machinery (rotary screw compressors, centrifugal compressors, large reciprocating compressors, gear boxes, generators, etc.).
The tube and shell design includes a bundle assembly of tube sheets, tubes, baffles and sometimes fins. This bundle is housed in a shell with heads that often contain flow dividers.
The hot air/gas is directed through the tubes while water or coolant is pumped through the shell. The fluid moves through a maze of baffles in the shell and removes the heat as it makes contact with the tube surfaces. In some applications, this process is reversed with the cooling fluid being pumped through the tubes while the product being cooled passes through the shell.
The majority of cooler problems are hidden and erode heat transfer effectiveness over time. It used to be a standard operating procedure to disassemble, clean and inspect coolers during an annual shutdown. This useful maintenance practice has been virtually eliminated with the financial pressures to maximize profit and avoid downtime.
The current "run to fail" mentality allows the hidden problems to worsen. Coolers are damaged beyond repair by deterioration of the baffles, tube sheets and heads. The ability to transfer heat suffers when cooling fluid bypasses the design flow due to eroded heads and tube sheets. The efficiency also drops when deposits form in the shell and contaminates plug the tubes.
Anodes and water treatment chemicals are relied upon to prevent some of the problems. These efforts can sometimes help, but they are poor substitutes for the annual cleaning and maintaining of coolers.
A by-product of the "run to fail" approach has been the proliferation of disposable tube and shell heat exchangers. These are built with fixed tube sheets making it impossible to remove the bundle from the shell for cleaning and repairs. These low cost units usually have a large number of thin wall, small diameter tubes jammed inside a small diameter shell. This increases the probability of fouling and leaking, reducing the useful service life of the cooler.
a) Tube and shell cooler manufacturers use an approach temperature when designing compressor coolers. The approach temperature is the difference between the temperature of the cooling liquid that enters the cooler and the temperature of the product being cooled, as it leaves the cooler.
Knowing the approach temperature gives you a way to monitor the effectiveness of your coolers. So, get the approach temperature for each of your coolers from the name plate or by calling the manufacturer.
Here's how it works on a cooler with a 15° F approach temperature: if the cooling fluid enters the cooler at 95° F, then the product leaving the cooler should be at or near 110° F, when the cooler is working properly. Corrective action is required, when the difference is not within a few degrees of the approach temperature.
Gather the approach temperatures for your coolers. Then, use a grease or paint pen to write the numbers on or near the coolers. This makes it possible for you to use a temp gun to quickly check the coolers during your daily rounds.
b) The flow of the cooling fluid should always be directly against the flow of the hot product being cooled, in a tube and shell cooler.
c) Avoid storing solvents, water treatment products and batteries near a tube and shell cooler being used on a compressor. The chemicals can be drawn into the cooler, mix with moisture from the air flow and create an acid that will damage materials often used in coolers (copper, aluminum and brass).
Send an email to coolers@CompressorWise.com if you have any questions or if you want to learn how to save money when buying tube and shell coolers.