A question on BTU's

[seymour]

In regards to plate exchangers, does refrigeration capacity btus equal heating capacity btus?
ex. advertised as 75k btu / hr refrigeration capacity = 75k btu heating capacity?

Also, things to look for when shopping for plate exchangers?

 

[arbutus]

The GEA flatplate heat exchanger website has a great calculator. You have to sign up (free) to use it, but I haven't had any spam from the company. Refrigeration capacity would be based on specific temperatures of water or brine or whatever the system is using, and you need to plug in a max and min temperature based on your system requirements.

 

[heaterman]

Short answer ....no.

The capacity of any heat exchanger regardless of type depends on two things. The temperature difference from side A to side B and the flow rate of the fluids moving through it.

What are you trying to heat with it?

 

[jebatty]

Claims of btu capacity on a plate hx are largely meaningless. Heaterman is right, and then add to that pressure drop on A and B for sizing the circulator to move the gal/min you need for the btu's you need to move. Use the calculator or use the published tables to get close.

 

[seymour]

Short answer ....no.

The capacity of any heat exchanger regardless of type depends on two things. The temperature difference from side A to side B and the flow rate of the fluids moving through it.

What are you trying to heat with it?

I'm using it to go from an unpressurised outdoor boiler to a pressurised infloor/cast rad house heating system. found some on amazon that are listed as being for beer wort cooling but will these work? any suggestions?

 

[jebatty]

Your question is a bit like "I'm taking a road trip and I found a car in the for sale ads. Will it work?" Arbutus advised to look at the GEA flatplate heat exchanger website calculator. When you collect the info needed, the calculator will tell you which plate hx's will work. Can you get some specs/advice from the seller/installer of your new system?

 

[heaterman]

I'm using it to go from an unpressurised outdoor boiler to a pressurised infloor/cast rad house heating system. found some on amazon that are listed as being for beer wort cooling but will these work? any suggestions?

So if you're using it for heating your house, the first question is what is the heat loss or btu load of your house? When you determine that, step 2 would be to figure out the flow rate you can get out of your underground tubing [length, diameter and pump you are using]
Step 3 is what water temp we have to work with in the outdoor boiler.

Then you can start to get some valid input on what size HX you actually need. You'll probably want to keep Side A pretty close to Side B temp wise in order to get the "approach temperature" very tight. This will keep your iron rads putting out maximum heat.

 

[seymour]

I would like the hx to be as close to 225k-250k btu as i can. Using 1" pex underground so i think thats about 8.5 gpm max? Boiler will run about 180-190*

 

[jebatty]

The parameters you state begin to get to what you might need. What type of wood boiler do you have? How much water does your boiler hold and how much additional hot water storage will you have, if any, Do you have on hand now or will you have on hand before you start burning an adequate supply of cut, split and well seasoned wood (20% MC, usually 1-2 years of drying, depending on the species but longer for some hardwoods). How much of well seasoned wood do you plan to have on hand? Green wood or poorly seasoned wood is the nemesis of any wood burning system.

Based on the parameters you state, delta-T will be in the range of 50-60F, is that what you are expecting? That is, 180-190F into the hx and 120-140F out of the hx? What is your coolest usable water out of the hx that will meet your needs? Will you have a mixing valve, and if so, at what settings? Do you expect the boiler output always to be in the 225-250k btu range, and 180-190F output range? That would be about 40-50 lbs/hr of wood in a very efficient boiler, or about one full cord every 3-4 days on a 24 hour basis. What do you expect your wood usage to be? What will be the distance between the boiler and the hx? Between the hx and you system manifold? What type of insulation are you planning for the underground pex?

Fairly normal boiler operations are based on a delta-T of 20F, and at 225-250k btu, that equates to about 25 gpm, far beyond the capacity of 1" pex, and pressure drop in a plate hx at 25 gpm would be very large and might pose quite a challenge in system design.

 

[seymour]

So i may have omitted some critical details. I will be using a pellet boiler for the heat. I am switching from a pressurised wood gassser to a unpressurised pellet unit. Located aprox 75' from hx. Thermopex used underground so already insulated. The advantage is that i will have a constant supply of hot water. Mixing valve at manifold mixing down to aprox 140*

 

[heaterman]

As Jim said, there is no way to get 200-250K out of 1" pex without something on the heat emitter side that can create a huge temperature drop.
Working with an optimistic 8GPM available through that length of 1" pex, each 1* of temp drop would equal 3998btu's. So to hit even 200,000 you would need something on the order of a 50* drop to generate that kind of load at that flow rate.

Only way you could hit that would be dumping 8gpm into a cold concrete slab. Supply to return on the boiler would have to show a 50* differential to soak up 200,000

8gpm x 8.33# of water/gallon x 50* x 60 minutes/hour = 199,920btu.

If those indeed are the parameters you need to hit, you will need a large heat exchanger to achieve that goal.

Using the following parameters this is what I come up with:

Boiler (side A) of the heat exchanger IN @ 180* ----- OUT (back to boiler) @ 160*, flow rate required = 20.5gpm
Load (side B) of the heat exchanger IN @ 120*------OUT (back to system) @ 175*, flow rate required = 7.4gpm
Heat exchanger size 5"x12"-50 plate with 1-1/4" ports will do the above.

Also should mention that the head developed by the HX will be .7 Ft. on Side B and 4.6 on Side A

So it's doable IF (big IF) your system can generate that kind of a temp drop. Once you start to decrease the T on the output or load side of the HX, everything goes out the window.
If your system can only generate a more normal 20* drop you will be limited to a range of 70,000 to 80,000 btu's due to the size of your pex.

The remaining question is....(drum roll).....what does your actual heating system consist of.
Coil in a furnace? Radiators? Baseboard? Heated slab?, Domestic hot water load? Combination of the above?

So to sum it up, I have to say that realistically speaking, you will not be able to transfer 200,000 btu's out of your boiler.

 

[jebatty]

I'm going to assume return water from the hx to the boiler will not be less than 140F, reason being most wood boilers need return water at this temp for return water protection to prevent condensation. If your pellet boiler has a different specification, what is that, and I will take another look. Then for your system, this is my first look:

Boiler supply 185F to hx
Boiler return 140F from hx (return water protection temperature)
Load 250,000 btuh
gpm = 11.11
1" Thermopex

Thermopex I believe has an ID of 3/4"
I don't know what the pressure drop/100 feet is for 1" Thermopex; you need to find out what that is, but I will use 3/4" ID steel pipe for the calculation.
Distance round trip = 150'
Approximate pressure drop/pump head = 29psi/67' pump head
Plate hx = additional pressure drop/pump head

At this time it not worth trying to spec a plate hx because the short answer is that if you want to move that many BTUs, you need much larger pipe than what you have. You need an appropriate system design.

 

[seymour]

Thanks for all the info. I understand the calcs but doubt i could have done it myself. You guys truely are a wealth of knoweledge. I dont actually need 250k btu to heat the house, its just that this the max output of the boiler. I want an hx that will give me max btu exchange with the given restrictions ie. 250k boiler , 75' of 1" thermopex. System contains approx 1000sq ft of wood floor joist infloor heat, 9 cast iron rads and dhw. Inside a average insulated two story + basement house.

 

[jebatty]

What is the water capacity of your pellet boiler? What rates of output do the settings on your pellet boiler permit? What is the spec'd minimum return water temperature for your pellet boiler? And I don't see a mention of what pellet boiler you have - will you tell us?

What will help considerably is a heat loss calculation for your house so that any advice given will be tuned to meet your needs. Designing your system to realize as many btu's from your boiler as possible, unless you need all those btu's, only will add unnecessary cost and complexity with little if any compensating benefit.

 

[heaterman]

What rates of output do the settings on your pellet boiler permit?
What is the spec'd minimum return water temperature for your pellet boiler?

These two are very important to get a handle on for the sake of your boiler.
If can only go down to a minimum of say 100,000 btu's it is going to either short cycle to death or else overheat without some form of thermal storage.

I'm guessing your average heat loss at near design conditions would be in the 60-80,000 range. More normal winter temps in the 10's-20's will put you at 50K or less.
If your boiler can go no lower than what I mentioned above, you have other (maybe more important) issues to consider besides the heat exchanger.

 

[seymour]

ok so boiler is a maxim 250 and has three modes low med and high also has auto relight. I quickly looked at the brochure and the website but it doesn't say what the actual modes equal percent wise. the boiler has 90 gal onboard and I'm planning on adding another 150-200 gal in the house but might not happen this year.
I have been looking into heat load calcs and everything I find seems very very complicated and detailed, I'm a building inspector so I have a thorough knowledge of building construction also I have all the details of my house, if anyone has an easy way or any way for that matter to calculate heat loads that would be great. thanks

 

[jebatty]

The EPA reports show the Maxim 250 has 212,453 btuh output over a sustained 4 hour burn. CB advertises 250,000 btuh and I would suppose that is the input rating. If so, about 85% efficiency. I too after a quick search cannot find any info on lower output rates.

 

[heaterman]

The EPA reports show the Maxim 250 has 212,453 btuh output over a sustained 4 hour burn. CB advertises 250,000 btuh and I would suppose that is the input rating. If so, about 85% efficiency. I too after a quick search cannot find any info on lower output rates.

Finding an accurate minimum input would be very beneficial in planning the system.

 

[seymour]

I think that it would be next to impossible to put a rating on the modes as they can be adjusted for fan settings and auger speeds so really each mode has an operating range that is set by the user.

 

[heaterman]

I did some checking this morning and could find no information on what the minimum firing rate is on these units either.