Chilled water question for Brad White

Brad White_185

Thanks, Bob- That makes sense.

Sort of one of those things you never thought about, like, "does the Queen of England go to the bathroom?"

Makes you go, "huh!"

:)

hvacfreak

counter flow versus ...

...whatever the oposite of that is. I recently had to re-pipe a number of ceiling hung ducted FCU's...( 4 pipe units that kinda remind me of horizontal residential air handlers ). I had to correct the piping ( the installers ) to the submittal information ...counterflow..???...coldest fluid entering the coil furthest from the inlet air. The coils are 4 and 6 row and typicly about 1200cfm air. Is there really a difference on these small units..supply and return connections I mean ? - Mike

hvacfreak

and I can now understand

why some folks really like Propress. This was my first time with it , and it made this rework alot easier , cleaner , no worries of smoke in a commercial building. Even with marking the full socket depth on each joint , it is the easiest piping system I have worked with. -m

jeff_110

Air on, water off

Standard mantra for cooling and heating coils, or for the left brained folks, water on, air off. For chilled water coils, the coldest water has to enter the air leaving side of the coil for most standard applications.

Brad White_185

Counterflow

Hi Mike-

The principle is the same regardless of the size of the unit. How these are circuited I do not know though. Some of the less expensive coils are manifolded such that all coil circuits see the same water temperature in and the same out more or less, so it negates the counterflow principle in part.


And no, I have no idea what the opposite of counter-flow is. Not exactly parallel but maybe it is in a way.

You have the counterflow concept correctly- the coldest water sees the coldest leaving air.

Say you have air entering a coil at the ubiquitous (and IMHO, uninhabitable) ARI standard of 80/67 and you want the air to leave at 54 saturated. Say also your CHW enters at 42F and leaves at 52F. Cool! (really :P)

Your initial difference in temperature where the air enters and water leaves is (80-52)= 28 degrees. Your leaving air and entering water gradient is (54-42)=12 degrees. The difference is enough to do some work or has the potential to do so.

If you reverse this, your initial approach is (80-42)=38 degrees, fantastic.

The leaving condition though is (54-52)=2 degrees. Not a lot of potential there and that assumes you can even GET to 54 degree air....

The previously cooled air (cooled by the first few rows) has diminished the delta-T across that coil. By front-loading your cooling you have placed a drag in efficiency on the leaving side. The last few rows have a temperature convergence at which point heat transfer stops.

In wide delta-T systems such as some districts use (16 or even 18 degrees) you can actually wind up adding heat back in those last rows if you do not use counter-flow. Figure a system using 44 to 60F water trying to make 54 degree air. You can start but you cannot finish!

bob_46

Opposite

I was doing some piping for some chemical engineers and they used the terms counter-current and co-current.

hvacfreak

I can see it..

...from your last statement. These were ( omit brand name ) "Blower Coil Units"...the customer complaints were performance based for certain. As it turns out , many had chilled water flowing through the 2 row heating coils and vice-versa. It made for an interesting correction without a complete drain of the building.

Had the submittal data spelled out the connections this never would have happened. I solved the riddle with the O&M model number breakdown / load data on the units , and coil connection spacing then became obvious.

And thanks all for the replies...and Brad for the 110th time , lol. -M

Eugene Silberstein 3

I show my students....

I show my students the concept using a water-cooled condenser with the water flowing in the same direction as the refrigerant. As the water and refrigerant flow through the condenser, the delta-t between the refrigerant and the water gets smaller and smaller until (theoretically) the delta-t disappears altogether and heat transfer stops.

This would be directly contradictory to the fact that larger condensers (all other things being equal) will result in more efficient heat rejection.

mtfallsmikey

Similar principle

Used in the Trane units in my buildings; the "Economizer" mode...instead of making chilled water, condenser water (open loop)is used to accomplish the same thing, water temps at 46-52 degrees. But it only works well when outdoor temps are below 34 deg. due to the heat load of the buildings. That was a better explaination than I had heard before, Brad.