Coil Energy Recovery Loops

[AO1958]

Hello Everybody,

I am drumming hardly in the calculation of a coil energy loop.
Just to explain, two coils put in the supply and exhaust air streams of a building or process.
The coils are connected in a closed loop by counterflow piping through which an intermediate heat transfer
fluid is pumped.

Does anybody of you know any literature reference for calculating this kind of application ?

Many thanks

 

[engair]

Why pump fluid? Just use a heat pipe, and it does all the work itself.

 

[AO1958]

Hello, many thanks
Unfortunately I cannot utilize your system, but I am studying this application

http://www.p2pays.org/ref/36/35285.pdf

in fig 1.

thanks for your help

 

[mauricestoker]

Sounds like a run-around loop instead of thermal siphon. Glycol mix is typical. I'm not a big fan of the run-around loops, mostly becuase I spend so much effort trying to separate and distance exhaust and supply (hospital and lab design).

Energy savings would be based on heating/cooling recovery versus pump power and duct static friction loss.

If first cost is not crucial, but life cycle is, I'd suggest looking at an enthalpy wheel. The glycol system will top out at limit of sensible heat, the wheel will pick up latent loading.


If you google "run-around" and "heat recovery" you'll get about 10,000,000 hits.

 

[coldmaster]

The effectiveness in terms of heat recovered will be based on actual coil selections, air temperatures and relative air volumes.

 

[ChasBean1]

Agree w/all that heat recovery can vary by application, but the order of ability to recover heat is typically 1) return air, 2) enthalpy wheel, 3) runaround loop, and 4) heat pipe, although 3 and 4 can have similar performance.

If you’re studying the exact application in Figure 1, it raises a question. Figure 1 shows a typical return air AHU that looks to use the runaround loop after the return for an unknown reason. If you can return the air (best possible heat recovery) why would you want to add a glycol runaround loop? The controls for this unit could be complex as the runaround loop heat recovery could conflict with the economizer.

The runaround loop is for labs, etc., where you don’t want the exhaust air stream touching supply, plus you'd prefer the exhaust AHUs to be separated from the supply AHUs.

My understanding is that runaround loops and heat pipes perform similarly, recovering sensible energy. Enthalpy wheels and return air systems recover sensible plus latent and are better options for relatively clean exhaust air systems.

 

[mauricestoker]

ChasBean,

Agree completely, with one caveat: the best energy recovery does not always guarantee energy savings. I'd be putting in 3A wheels left and right if it did. The less fun side still remains, engineering economics.

 

[ChasBean1]

Thanks Maurice, but I've no flippin' clue what you just said!! It sounded pretty cool though... 3A wheels, with maybe those spinny rims or something.

 

[mauricestoker]

I didn't know how to put the angstrom sign in.

The intent was that there are many different ways to conserve energy, they just probably won't be done if they do not pay for themselves over a limited amounbt of time.

 

[engair]

Maurice is right, engery recovery is the first thing I see VE'd out most the time

 

[mauricestoker]

I have 10 variable inlet guide vanes on VAV's that run below 40% capacity. VFD would be a no-brainer. Guess what got VE'd out? Showing the ASHRAE 90.1 requirements for turndown did not make an impression, the upfront cost was sent back as "too much", even with a very solid payback. Guess I need to work on my interpersonal skills.

 

[ChasBean1]

Maurice, you're right - unfortunately payback is so often inconsequential to those that have to write the checks up front.

 

[Zesti]

As the architect on any project to tell you the payback time on all those fancy cost-increasing things he dreams up...

It's usually HVAC that comes last when it comes to dividing up the total budget for a project.

 

[jOHn10]

A company by the name of Kathabar carries a product line for total energy recovery with no cross contamination of the supply/exhaust air streams; I believe its called twin-cel...

 

[mauricestoker]

John10,

I'm dfinitely going to check that out. Getting ready for two large projects in a med center, current criteria allows no latent recovery becuase of cross-contamination concerns, although they have an enthalpy wheel that has worked for 25 years with no indication of crossover.

 

[AO1958]

Many thanks to everybody !
to Mr mauricestoker

Indeed I have googled "run-around" and "heat recovery".

Nevertheless I am not able to find any reference concerning the calculation of these systems.

please, has anybody any hint concerning this issue ?

many thanks

 

[cdxx139]

Some manufacturer;s, like Greenheck have selection programs for their energy wheel that also give payback analysis built in. You update the defaults to your area.

Some manufacturer's allow a purge that pressurizes the clean side so that an cross contamination goes from clean to contaminated side. Of course this may not always work correctly, and you will have to get owner acceptance, but the energy savings of latent recovery is great.

You should reconsider the heatpipe, if possible. The can now be installed if the duct is side by side or 10 feet apart. The runaround requires the pump, and hydronic equipment such as expansion tk, etc. Higher first cost and more maintenance.

knowledge is power

 

[ChrisConley]

To answer the OP (although from the age of the post I'd assume they have disappeared) manually calculating a run-around loop is very difficult (and iterative).

The last time I did a run-around system for a retro-fit I contacted Engineered Air. They have a selection program that does the work of calculating the coil sizes and effectiveness.

Good luck.

 

[cvanoverbeke]

ASHRAE Applications Handbook has details of the calculations. We just finished a design of a glycol run around loop. The system effectiveness is driven by coil selection and this is related to how much energy the coil can transfer at the temperatures you are using.

 

[AO1958]

Dear Mr. cvanoverbeke,

please it would be extremely helpful for me to have the exact reference in ASHRAE applications handbook.
As a matter of fact I jumped into the book I have available but I didn't find any reference.

Many thanks