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It cools the blood in the major back/front muscles, which is like a big radiator, right?  Then you drink it.  

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Couple of tips to help the OP out.

1.  The cummerbund for the plate carrier should be attached as low as possible on the back plate bag and as high as possible to the front plate bag.  This will push the rear plate up, which will provide better coverage for your back and better weight distribution/comfort on your back.

2.  High Temperatures mean you will breath heavier and that means more expansion of your chest cavity.  A tight plate carrier is a comfortable one, but a tight plate carrier reduces your ability to breath heavier/more efficiently in high heat. The solution to this problem is an elastic cummerbund for high heat/high mobility.   The elastic cummerbund will hold the front and rear plate bag tight and in place while being able to expand while you breath deeply and heavily.

3.  Check out Qore Performance Ice Plate Pontoons/Vents, whatever they call them.  In short, pontoons that you sew into or attach via already available velcro on your front and rear plate bag will increase bulk, but will also provide a channel for air to flow somewhat reducing the retention of heat.

4.  Hydrate.  You will sweat more heavily in a plate carrier and you must adjust your water consumption accordingly. If you want help figuring out how much water you should be drinking get the sweat test strips from Gatorade, pretty simple way to questimate what your water consumption should be based on your sweat loss (some dudes sweat heavier than others).   There are more sophisticated ways of calculating your water consumption requirements that Tri-Athletes use, but the Gatorade strips are cheaper and I think decent unless you're going all out to do an Iron Man or something.

5.  20-30 LBS is no joke. You're in the obese category and don't feel bad about it society is set up to encourage you to be obese.  You've got o do something about it though, so instead of investing more in ammunition or fancy gear it's time to invest in yourself, your core 1.0 kit, which is your body.   I would do the following:

A.  Hire a nutrition coach that will help you calculate your calorie and macronutrient consumption. There are many options, they range in price and in level of service provided.  I have used a company called StrongerU in the past, but now I do it myself because I learned how to do it after two years of being coached in it.

B.  Hire a personal trainer.  You need to discuss with a certified personal trainer your physical issues (bad shoulder, bad knee, and so forth) and you need to come up with a realistic exercise routine you can consistently maintain.  Stop "show lifting", stop doing the same workout you followed after reading a Muscle and Fitness magazine in high school.  Make sure you're using good form with a second pair of well trained eyes on you.  I'm not saying you have to be Hollywood and have a trainer for every session here on out, but at least do a few sessions with a trainer to dial in your form. Avoiding injury and keeping consistently training will unlock your ability to get in shape.

C.  If you're an extravert look for a gym that is highly social with perhaps good group classes to help. If you're an introvert get some home gym equipment like a Rogue Power Rack, Assault Bike, Concept Rower.

D.  Learn the value of compression sleeves and rehabilitative massage.  Learn how to foam roll. Learn how to stretch, yoga isn't a bad idea.   Give your bones time to harden and toughen since you're placing 20-30 lbs more on them than they're basically rated for.  Don't run on hard concrete surfaces, get good running shoes, trying incorporating things like swimming laps into your workout routine.  Remember growth happens when you rest properly after a good workout so the rest period is absolutely important.  Saunas and cold tubs can be extremely helpful as well.

Just remember, you're an important part of the security of our free state.  It is your civic duty to be capable of serving in a light infantry role in a well regulated unit if called upon to do so.  Train, eat, exercise, and equip yourself accordingly.