Sandbath and Hot spring in a day?

Hi all,
Does it sounds silly to do Hot spring in Yufuin and visit beppu for Hells of Beppu and after that to visit the sand bath in beppu?

Can this be a half day event (reaching about 10-11 and leaving about 4) and if one can take the heat doing both hot spring and sandbath in a day?

How the hot spring sand bath works? Is it an area with manys shops to choose from and pay to use the facilities at a given time? Do we need to bring anything additional?

Thank you.

Does it sounds silly to do Hot spring in Yufuin and visit beppu for Hells of Beppu and after that to visit the sand bath in beppu?

No.

Can this be a half day event (reaching about 10-11 and leaving about 4)

No, I think you need more time. Maybe if you were to travel by car and limited yourself to just one or two of the hells, you would be possible.

and if one can take the heat doing both hot spring and sandbath in a day?

Yes, that should be no problem.

How the hot spring sand bath works?

There is an indoor one in an old buildings and an outdoor one at the beach. In both cases, you will change into a light yukata, and then lie into a small hole in the sand. The bath staff will then shovel warm sand onto you until only your head looks out. It is then up to you to stay as long as you wish. It is recommended not to "bathe" for more than 10-15 minutes. On busy days, there might be a waiting time. You do not need to bring anything.

Thanks Uji.

In that case i will stay a day in Beppu.

May i know typical how long will it takes for the all the hells visit? about 3-4 hours? i will be using public transport.

I should still be able for the bathing facilities near evening?

Where are you going from the sand bathing? Back to Yufuin?
You can stay in Yufuin, take onsen early in the morning before bf when empty & all to yourself, great at sun rise. Take quick & frequent Kamenoi local bus to Beppu. There are frequent buses to the hell holes. You can miss some of them, i.e. crock cage, etc. After that go to the Beppu port and walk along the beach to find a sand bath hut. All can be done in a half day.

Visiting the all is time consuming and not really worth it if your time is restricted. Total visiting times widely vary on individual travelers. If you do it by public transportation, I would estimate typically 2-3 hours, starting and ending at Beppu Station.

But again, I don't recommend to see all. For example, you could limit yourself to the ones around Kannawa. That probably saves you 30-60 minutes:
http://www.japan-guide.com/e/e4702.html