Hydronic System

[Nick Badame Refrig/ACC]

Also,

As far as chilled water in a bus, seems to be the long way home for such a small area in my eyes.

Nick-

[Tim Strommen]

Several facts conspire to make 'chilled water' cooling with the same heat exchangers used for heating difficult.  Let's use an example of one of the 'toe kick' heaters in my bus.  It is rated at 7,000 BTU/h in heating.  This rating is with 180 degree water heating 70 degree room air (we note a 110 degree difference). When we try to use it for cooling  about the coldest we'd expect the water to be would be, say 40 degrees (the temperature difference is only 30 degrees).  Well, it looks like we've only got a little more than 1/4 the temperature difference and that's going to cause about the same ratio of heat exchange capacity.  Of course we can overcome this by installing FOUR TIMES the heat exchange capacity.

Perhaps another and more cost/labor effective way to get better performance out of a cold water system, is to use a lower temperature for the cold water.  With antifreeze, the freezing point can be reduced to a bit lower than -20degF (common AC Delco "Dexcool" mix can go to -34 before freezing).  A roughly 20% salt/water "brine" in the ice box would have a phase change at about -21degF.  By cooling the "brine" solution into an ice block (to about -25degF for good measure) we can get a sizable mass of ice at a low temperature (through a phase change) which can absorb heat energy from circulation coils in the ice block while it melts at a steady temperature until it's all liquid - at which point it will continue absorb heat energy as the now "cold liquid" starts to raise in temperature until it is the same temperature as the room.  To get the same performance out of those toe-kick heat-exchangers, you were looking for a 110degF difference which with a starting point of 70degF would be -40.  Getting near this is do-able with a proper thermal fluid - and if one selects the same mix for both the prime-mover and the HVAC&R system, the procurement should be easier (fewer discreet fluids to obtain/stock).  By regulating the flow of circulating water through the ice block, one can control the BTU absorption of the cooling system.  The rate at which the BTUs will be absorbed will also slowly decline as the ice turns into water and eventually heats up to the room's temperature.  With proper design and managment of the cooling capacity of the system, it could run all day on a hot day and charge all night.

This is also the same method mentioned in the previous post, and Kyle's post about the "eutectic" or "brine" or "anti-freeze solution" which is frozen into ice within a container and allowed to melt slowly in order to absorb the ambient heat from the surrounding cold-box.  The big killer here is the way that the ice absorbs the heat energy - the calculations speak of temperature and mass (the word "mass" should be a sign about what we are referring to here...).  In order to sustain this method over a given hot day, there would probably be a need for a rather large block of "ice" in the morning (think a few hundred/thousand pounds - a gallon of water weighs about 8.34lbs, 100 gallons is darn near 1000lbs), and the space this would take would probably offset its benefits in most if not all installations.  The other concern might be the cold water in piping - the extremely low temps would limit your building material selection severely.


[Edit]   A pound of freshwater ice (about 1/8th of a gallon) requires about 143.3 BTU to melt it - so you would need to size your system based on how many BTUs you intend to pull from the room over a day.  If you have a bus which needs 30,000BTU to keep cool (a common example - the equivalent of two 15KBTU roof-airs), and must be run for 8-hours, you need roughly 8Hours x 30,000BTU = 240,000BTU (20Tons) stored to sustain a day's worth of cooling.  Assuming a perfect insulation on the ice tank and no losses on the piping (BIG assumption ), this works out to roughly 240,000 / 143 = 1680lbs of ice (or 1680lbs / 8.34lbs per gallon = ~200Gallons of water/ice storage).  And remember you can't drink this!

One other point I almost missed - The above is the melting cycle over 8 hours.  The refrigeration system must still freeze the water once a day, so it must be able to sustain the cooling loads and the freezing load during its run-cycle.  It takes a lot of power to put out 20Tons of cooling in a few hours (6-8hours).  The size of this system alone may require the continuous running of the refrigeration all day (or 100% duty cycle) with lower capacity (i.e. 10,000BTU all day long) to be able to keep up with the total system demand. [/Edit]

Cheers!

-Tim

P.S. - I saw this product (the "Ice Bear 50") in use in Arizona on a "green-tech" show recently.  It used the discussed methodology of ice storage during off-peak hours and coolant circulation during peak demand hours. -T

[Nick Badame Refrig/ACC]

Hi Tim,

Great analogy!   

About 15 years ago, a customer of mine wanted to air condition his house with his boiler and base board heat. When he asked me to do this for him, I told him

that I was Not intrested! A year later, he called us for his annual boiler maintenance. Much to my surprise, I saw a 7 1/2 to condensing unit piped to a water chiller

exchanger that is used for "lobster tanks" to chill the water in his boiler.  I couldn't believe that he actually did this to his system. I asked his wife how it worked

with a chuckle under my breath.  She replied with a great big grin... That SOB ruined my hard wood floors!  If you could only imagine how much condensation

dripped from the base board radiators onto the floors the entire summer..... This is when I came up with the phrase, "Cheap Pays Twice" which I commonly

use alot when I see goofy things like this. That winter his wife contracted with us to install two high efficency ducted systems.

Nick-

[Tim Strommen]

 

"Cheap Pays Twice"

Ah - as a lower "peon" in a technology company, this is a difficult concept to explain to people who are looking at the immediate pay-off for cutting a corner.


They're thinking: "If we don't do this time consuming and expensive thing now - we'll save money!"

While all of the Engineers and support staff around the table are thinking: "You're right!  Except in a few months, we'll have to undo everything we're doing now and do it right (the way we just told you we were going to do it from the beginning) at double the time-investment and cost - plus the interim expense of our comapny's image! Go Team! "

Cheers!

-Tim 

[kyle4501]

Me thinks some have missed something. 

I thought everyone knew that A/Cs are most efficient when the compressor runs constantly. 

100# or so if ice could act as a buffer on hot days. Could also take care of quiet time cooling needs. 

As with anything, you need to evaluate YOUR needs/ wants before you make a decision.

As for the condensate water ruining the wood floors when chilled water was used on the base board radiators - that is what happens when one acts without consideration (or understanding) of the whole picture.

If one was to persue the chilled water thing, I'd assume they'd also pay attention to the value of insulation so you wouldn't normally need 30,000 BTU/hr for cooling.

The converted bus is made up of many systems & looking at how some of these systems work together is what this thread is addressing.

The chilled water project is definitely NOT for those who like their standard 'off the shelf' sub-systems.