Forced Air Circulation vs Block Heating Column Oven

[syx]

Dear Members,
What are the advantages and disadvantages between forced air circulation and block heating column oven? Which is better? I hope I could create a pool here to know which type used more widely.

Best regards,
Siswanto Tanuatmojo

[tom jupille]

Air circulation gives you more flexibility in column geometry and placement. Block heater provides better heat transfer.

[DR]

better heat transfer=faster equilibration times

Block heaters can be tougher to check with thermocouples though. You have to play with them and just see what works.

[Mark Tracy]

I wish I could find the reference. I read a report years ago that compared column temperature control designs for temperature uniformity. To my surprise, the forced air design had better spatial and temporal uniformity than the block design. (Also better than convection, but that was no surprise.)

There are a lot of subtleties to a good block design, and a poor design will actually have worse heat transfer than forced air. Sorry to say, I inflicted one such thing on the world.

A related issue is preheat of the mobile phase. I have been testing a method at 50 °C, and for a 4.6mm column at 2 mL/min it nearly doubled the plate count. For a 2.1mm column at 0.5 mL/min, it nearly halved the count.

Here at Dionex, our better lines use forced air, and have a preheat option. The low-end lines use a block, also with a preheat option.

[Srinivas SK]

Forced air heaters are better by far. Block heaters are usually cheaper but their performance is in no way comparable to a forced air system.

FA convection heaters attain set temperatures quickly and are more accurate than block heaters. And they are validation-compliant. They cost more but they're worth the investment.

[tom jupille]

I'll agree with Mark: there are good and bad examples of each, and heat-exchanger design is critical. John Dolan's "LC Troubleshooting" column in LC-GC magazine has dealt with this issue a number of times (these references are for the North American edition):

LC-GC 14(11) 944 (1996) In this case, there was a 13-degree effective temp difference between block and air-bath heaters from the same manufacturer - the air-bath heater was lower.

LC-GC 16(12) 1080 (1998) Looked at recommendations for preheater tubing length as a function of flow and temperature.

[sjmarin]

I think a good forced air oven will provide better heating that is more even and will equlibrate more quickly if the design is good. It will also provide some preheating, but not enough to avoid thermal mismatch above 40°C with normal length connection tubing. If you plan to do HPLC above 50°C, you need to address preheating.