Winterization of Coolers
With the approach of winter some operators face the recurring problem of freezing and hydrate plugging in their aerial coolers. Units sensitive to this problem are compressor inter and after coolers in undehydrated gas service, particularly when operating in turned-down mode. Typical hydrate formation temperatures are in the range 50º to 60ºF. However higher pressure may elevate this temperature to 70ºF+.
The practical problem with compressor aftercoolers is that it is not sufficient to merely maintain the bulk outlet gas temperature above the hydrate point for two reasons.
- Firstly, the tube wall temperature will be lower than the bulk gas outlet temperature, and
- Secondly, individual tubes will overcool the gas to a temperature which is not sensed by the outlet TIC.
The tube wall temperature typically settles at some value between gas outlet temperature and air inlet temperature and this dictates whether or not hydrate will start to accumulate inside the tube. Once deposition begins, the situation tends to runaway as heat transfer from the gas to the tube wall is reduced.
This causes a further drop in tube wall temperature and the more deposits etc.
Remedies
The standard air cooler industry remedy for winterization is the warm-air recirculation chamber as depicted below. Process temperature controls the top louvres (A) whilst an air averaging sensor controls the recirc louvres (B) and usually one set of intake louvres (C). There are good and bad louvre designs, good and bad ways to operate them, and right and wrong locations for the air temperature sensing line. A good louvre design will have tight closing inlet and discharge louvres and a sizable downdraft chamber cross-section (at least equal to ½ the total fan area) so that an adequate warm air volume is forced to recirculate. The air temperature averaging sensor line should not be installed between the fans and bundles but rather in the air mixing zone at the bottom of the recirc chamber under the fans. Otherwise, this can lead to unstable control feedback when operator shuts off one of two fans in winter.
Very hot air, which has passed through the bundle twice, passes across half of the air averaging line. This fools the controller into believing that the air temperature into the bundle is much warmer than it actually is on the half where one fan is still operating. The controller then incorrectly closes the down-draft recirc louvres. As the top process louvres (which are usually controlled by outlet gas temperature) are firmly closed this forces more hot air down the "dead" fan cell which further compounds the air sensor error causing the air intake louvres to open wide whilst the operating fan continues to blast cold air at one half of the cooler.
Call us at 274-5882 for further information on the winterization benefits of VFD in regard to optimizing tube wall profiles and resulting hydrate inhibition using VFD controlled coolers.
This problem where the air sensor is located above the fans is illustrated below.
Relocation of the air averaging sensor usually remedies this problem. Operators with the luxury of two speed fans or autovariable fan pitch are better advised to keep all fans operating at low speed or low pitch rather than alternating fans off and on across the cooler.
Operators with the luxury of variable speed fan drives (VFD) should run all fans all winter and allow rundown to 10% or less as necessary in -40ºF weather. This also maximizes the winter electrical savings. For example running half the fans at full speed and half shutdown saves 50% power. Running all fans at 50% speed saves 87% of the power due to the fan speed/power laws.
