Heat Exchanger Sizing

I don't see the water temperature when it enters the heat exchanger. From what I see here it is at 75°F already.

I waved today when I went by on I-29.

Hello good bye, yep I don't know that temperature the facility I think will be operational later than I'd hoped, probably March or April, hoping to get the footings and stem wall poured before turkey day. I'm expecting temperature specs from our system consultant soon, I probably seem a bit antsy.

Does the water ever leave the greenhouse?

How much heat is added by fish, bacteria, pumping, exothermif chemical addition?

How much heat is lost to evaporation, endothermic chemical addition?

If water never leaves the greenhouse and the greenhouse is maintained at 75F....I'm not seeing the need....oh wait, are the fish tanks not within the greenhouse?

The entire system is inside the greenhouse and there is no water discharge, except for that water contained in the harvested produce.

Only premium fish food is added to the water, no chemical fertilizers and no pesticides. The nutrient stream is derived from the activity of bacteria. The water laden with fish excrement flows by gravity through the several sediment tanks where heavy solids are separated and a bacteria consumes the ammonia and excretes nitrite. The flow then moves into another sediment removing media tank and another bacteria consumes the nitrite and excretes nitrate which flows into raft tanks for uptake by plants.

What amount of heat is lost through transpiration is an unknown at this point.

In order to maintain a 65% humidity venting at the roof peak and each side is necessary.

I know I am probably to late to the party,but I think an air to air heat exchanger that would recover the heat and control humidity would be a much more thermally efficient approach to controling humidity.

I built one for my Aquaponics greenhouse in Vermont it keeps the humidity at 55 % and the incoming air is only 5 to 10% below the 70deg. Inside temp.

it consists of 21 layers of metal roof material, spaced 1/2" apart 39" wide and 6' tall .

the warm inside air travels down through 10 of the 1/2" gaps and condences the humidity as it is cooled . The water is colected and returned to the system

the fresh air is drawn up through the other 10 gaps between warming it to nearly the inside temp. It is powered by 2 hi efficiency 300 cfm fans , 1 blowing out and 1 blowing out.

Obviously you would need a or several larger units but it would save an enormous amount of heat.

From what I gather, your running about 50 gpm, you need to maintain 75F-83F that is passing through a 1500 gallon buffer tank heated at 115F.

what type,of HX is in the buffer tank, is it piping, finned piping.

and after reading it over to extrapolate the information and glean enough information on what you want.

It sounds like this a proposed.

• Do you plan on building this or contracting it out? did you talk to an engineering firm? They can run the calculations for this.
• If your doing it on your own. A lot of variables can happen that effect efficiencies, to get your heat transfer area,
• as what size of piping?, is it small enough for turbulent flow for heat transfer, if it is, temperature can be regulated by the flow rate.
• what is the material of the piping and wall thickness?
• is this buffer tank recirculated?, is it an open buffer tank? how is it heated? Is it sufficient to cover its losses?

you gave quite a bit of info on the process, I ignored what I didn't understand such as rafts..

The raft is displayed holding plants as the roots dangle in the nutrient stream

Ok, I see, hydroponics, Nice.

Piping is 6" schedule 40 pvc.

I'd thought to keep water temperature below 130F reducing scale but using soft water seems a better segway. If I circulate soft water through the boiler loop 180F to 1500 gallon buffer tank and 201,900 btu air handlers with 70F input temperature resulting 100F output temperature 4650 cfm, 47' throw when thermostat calls for heat.

The hydronic engineer didn't care to hear all the pertinent info and scurried off to determine the load so thought to gather knowledge at CR4.

6" is pretty big for 50 gpm, not going to get very good heat transfer with laminar flow

OP said 3,000 gph.

Did I miss something?

Yes, the conversion factor from hours to minutes.

50 gpm is 3000 gph,

Its habit for me, I use gpm for laminar/turbulent flow calculations.

Oops. I'm stuck at gph.

The pump is below the HX so the pipe will be full; and then the water is returned to fish tanks that are at the highest point in the system.

My point is if it's a laminar flow, the Boudreaux layers against the wall actually acts as an insulator to the core of the 6" pipe..

now I don't know how long your pipe is, or how many elbows. One thing people use to get a turbulent flow on heat exchangers is to use turbulators to get the to break up the boundary layers for a mixing action for the flow in your heat exchange.

Pipe is 10' and can be 4"

I used an online app for determining turbulent flow.

I didn't think you'd have it, but your did.

But if your pipe is only 10' in length, your dwell time (about 6 seconds) for a thermal exchange is not long enough.

The six inch pipe carries water to the HX.

I see, again is this still in the design stage? if it isn't what do you currently have.

Yes our infrastructure is a slight departure from the expectation of the systems basic design.

awesome project JD. I wish you the best of luck, hope to sample your fish and lettuce soon!

My experience is in collecting, storing, distributing one sporadic stream of BTU's, while distributing them into an independent demand stream that has it's own steady state thermostatic requirements (like your greenhouse) whilst immersed in a third temperature environment (the weather) Not having a substantial insulation envelope makes it much more challenging.

HVAC guys/Boiler techs can be really good at this, or lost, because they often deal with steady state BTU production, which you will have with your boiler. Engineer the most adjustable, upgradeable and cost effective system you can. It appears that you have a handle on the data required. I would consider a geothermal heat pump. You may be able to alter your heat ex values to much lower temps, which generally increases efficiencies. A body of water like a lake or large pond is a good sink.

Thanks I was thinking so and expect to follow the German example for sizing the buffer tank and put a additional 10,000 gallons underneath as this will aid the systems for the next four greenhouses too.

heat loss on a big volume, even really well insulated, can eat into your energy budget. I don't see the need for a large buffer, what is your line of thinking there?

I've decided to make a side arm HX. The six inch pipe I'll run to a sump and immediately after the sump will fit a pump into 2" pipe that'll run through the shell HX. Now to figure the length of HX.

Check out Cornell Univ. Hydroponic Lettuce Handbook. On my phone so I can't send link.

Sounds like a very interesting project.

When it comes to choosing your heat exchanger may I suggest SWEP brazed plate heat exchangers. I use them for glycol / refrigerant applications and their technical support is excellent. Plus you can use their software and play around with different feeds etc.

No other endorsement here other than I have had good experience with them.

Good Luck!

Thanks and nice byline

Having had a 70,000 sq. ft., greenhouse with waterfalls, aquaponics, the whole deal much like yours basically - only I wasn't producing much, just retail and custom insulation, but I learned something that I believe will give you some insight.

No matter how well you track data, plan, etc., between the unpredictability of the weather, and (I didn't see this one coming) all the interactions chemically, biologically and perhaps the night fairies for all I know, give and take calories, and it's just asking too much to "equate" with any better results than a couple experienced people and common sense. The heat given off and used in this ecosystem is quite a large factor.

You probably need to figure a Murphy in there too as I see your location - one bad night and we could have another catastrophic event!

Fully agree and our backups have backups have backup but a compromised envelope we've also got provisions for unless I can't close the hole. I'll be there within six minutes but if the crop is lost the 38 day cycle will begin again, however if the fish are lost I'll have to raid the fish nursery.

This's our adventure too

Very good insight Misc, OP I suggest you take notice. A green house, by definition, uses solar gain, which is highly variable, and non correlated with outside temperature, driving a very dynamic heat cycle, far more than typical exchanger system engineering is used to, in my experience. Your system must be highly variable in order to operate efficiently. Sensor driven boiler flow rate, boiler temperature, humidity, venting apertures and fan speeds will need to be adjusted by a very smart program and a very well informed person.

I don't get the concern over the 6" PVC lines, you are not suggesting that for your internal HX material, are you? That will be ideal for fluid transport, as it will afford you very low head losses.

The main heating concern is for nighttime and heavy clouds, there will be a greenhouse control computer.

This is a good basis upon which to approach heat exchanger manufacturers directly for quotations. All the best with that mission.

You are planning to build the heat exchanger yourself? Any hxr manufacturer could calculate this for you and build it.