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Loop Temp. Drop?
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RB_2
Member Posts: 272
Some very general thoughts for you to consider
.
The room load will pull down the fluid temperature as long as a delta t exists between them (theoretically if the fluid flow is trickling very slowly through the pipes at some point the outlet temperature could approach the temperature of the room.)
The R value of the floor will resist the speed of transfer. The higher the R value the higher the fluid temperature required and or the closer the tubes to maintain a design floor surface temperature.
The flow rate contributes to the average fluid temperature which is the theoretical driving force to keep the floor surface at design conditions. A narrow delta t drives up the average, a wider delta t drives down the average.
A higher average fluid temperate (given a fixed inlet temp ) translates to wider tube spacing but requires more horsepower; a lower average translates to narrower spacing using less horsepower. Using a wider delta t and boosting the inlet temperature should be considered to optimize designs.
Loop lengths primarily impact differential pressure a competent designer can play with temperatures, pipe diameter (fluid velocity), and tube patterns to overcome challenges of short or long loops. The caveat to this is when a circulator is improperly selected. A designer may calculate for a 20 degree delta t only to find out later the installed circulator was undersized and incapable of delivering the flow at design conditions ie: the head loss was too great resulting in a lower flow and thus a drop in the average fluid temperature or perhaps the right circulator was installed but the loop lengths were considerably longer than designed.
Different types of radiant floors require different designs.
An 8 thick concrete slab can easily use wider delta ts especially if the tubes are located towards the bottom of the slab. Tube patterns can equalize the surface temperature variations if necessary. A 1.25 light weight topping system on the other hand benefits from narrower delta ts to maintain constant floor temperatures. The floor covering will have a major impact on this. A tile floor resist heat transfer less than carpet so one could potentially notice a different surface temperature if the tubes are too wide or flow rate to slow as a result of designing for large delta ts.
Of course as Hot Rod has pointed out there is a difference between steady state vs dynamic state. Steady state is used in designing a system for maximum load but we know this rarely occurs because the load is always changing (dynamic). Cold weather start ups are a good example of where steady state design parameters are challenged.
In summary, the topic is really an intellectual exercise ( mental masturbation as per Bill Wrights commanding officer) designers select a delta t to determine the flow understanding that at maximum load conditions the average fluid temperature will be maintained and deliver the required floor surface temperature to maintain comfort conditions .should the load exceed the design parameters the average fluid temp will drop thus reducing the speed of transfer resulting in a drop in room temp if the load is less than design the average will rise thus improving the speed of transfer.
As Hot Rod pointed out .software is the best way to play this game but if you really want to do the hand calculations pick up a copy of ASHRAE 2000 HVAC Systems and Equipment manual, section S06 recommend you surround yourself with a calculator, pads of paper, pencil, sharpen, eraser, a large pitcher of water and bottle of Tylenol.
rb
The room load will pull down the fluid temperature as long as a delta t exists between them (theoretically if the fluid flow is trickling very slowly through the pipes at some point the outlet temperature could approach the temperature of the room.)
The R value of the floor will resist the speed of transfer. The higher the R value the higher the fluid temperature required and or the closer the tubes to maintain a design floor surface temperature.
The flow rate contributes to the average fluid temperature which is the theoretical driving force to keep the floor surface at design conditions. A narrow delta t drives up the average, a wider delta t drives down the average.
A higher average fluid temperate (given a fixed inlet temp ) translates to wider tube spacing but requires more horsepower; a lower average translates to narrower spacing using less horsepower. Using a wider delta t and boosting the inlet temperature should be considered to optimize designs.
Loop lengths primarily impact differential pressure a competent designer can play with temperatures, pipe diameter (fluid velocity), and tube patterns to overcome challenges of short or long loops. The caveat to this is when a circulator is improperly selected. A designer may calculate for a 20 degree delta t only to find out later the installed circulator was undersized and incapable of delivering the flow at design conditions ie: the head loss was too great resulting in a lower flow and thus a drop in the average fluid temperature or perhaps the right circulator was installed but the loop lengths were considerably longer than designed.
Different types of radiant floors require different designs.
An 8 thick concrete slab can easily use wider delta ts especially if the tubes are located towards the bottom of the slab. Tube patterns can equalize the surface temperature variations if necessary. A 1.25 light weight topping system on the other hand benefits from narrower delta ts to maintain constant floor temperatures. The floor covering will have a major impact on this. A tile floor resist heat transfer less than carpet so one could potentially notice a different surface temperature if the tubes are too wide or flow rate to slow as a result of designing for large delta ts.
Of course as Hot Rod has pointed out there is a difference between steady state vs dynamic state. Steady state is used in designing a system for maximum load but we know this rarely occurs because the load is always changing (dynamic). Cold weather start ups are a good example of where steady state design parameters are challenged.
In summary, the topic is really an intellectual exercise ( mental masturbation as per Bill Wrights commanding officer) designers select a delta t to determine the flow understanding that at maximum load conditions the average fluid temperature will be maintained and deliver the required floor surface temperature to maintain comfort conditions .should the load exceed the design parameters the average fluid temp will drop thus reducing the speed of transfer resulting in a drop in room temp if the load is less than design the average will rise thus improving the speed of transfer.
As Hot Rod pointed out .software is the best way to play this game but if you really want to do the hand calculations pick up a copy of ASHRAE 2000 HVAC Systems and Equipment manual, section S06 recommend you surround yourself with a calculator, pads of paper, pencil, sharpen, eraser, a large pitcher of water and bottle of Tylenol.
rb
0
Comments
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I understand the formula to get required flow in a loop (BTUHr.|490 X Temp difference of Supply&Return). But how do you know what your Temp difference between supply and return will be? I've never really heard it being discussed and figure it would have to depend on loop length and floor temp. How is this calculated? Thanks.0 -
It changes
If you were to start a 32F slab up on a cold day you may see 40, 60 or more delta T. As the slab, and space warms the delta T will drop. The difference could drop to 3-5 as the slab, and room temperature nears it's setpoint.
When you do a design you can play around with "desired" delta T. Generally 15- 20 is selected, for residential. Large shop spaces may be pushed to 30 or more.
This will be determined by the flow rate through the loop, pump size, tube diameter, loop length, surrounding air temperature, all play into the formula. Grab a heat loss and design program and play around with inputs and see how they work with one another.
hot rod
To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
When you size a pump for a certain BTU load, wouldn't you have to know the delta T first? Isn't that how the pump capacity is selected (besides the loop head)?0 -
yes, sort of
You find out from your BTUs and your Temperature drop, how many GPMs you are needing to move.
You measure the resistance of the pipe run (head) and the GPM to plot on a graph that shows which pumps can do the job under those circumstances.
Noel0 -
Suppose you design called for
a 7gpm flow at 7 feet of head. Which pump would you choose from that sheet?
A system curve overlay could be used to plot the point where the pump will actually operate. It it helpful when several pumps all fall close.
Notice one pump on that sheet stays within a close gpm range over a wider range of pressure drop.
hot rod
To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
So- you would have to know your Delta T before you select your pump, correct?0 -
dt's
Some very general thoughts for you to consider .
The room load will pull down the fluid temperature as long as a delta t exists between them (theoretically if the fluid flow is trickling very slowly through the pipes at some point the outlet temperature could approach the temperature of the room.)
The R value of the floor will resist the speed of transfer. The higher the R value the higher the fluid temperature required and or the closer the tubes to maintain a design floor surface temperature.
The flow rate contributes to the average fluid temperature which is the theoretical driving force to keep the floor surface at design conditions. A narrow delta t drives up the average, a wider delta t drives down the average.
A higher average fluid temperate (given a fixed inlet temp ) translates to wider tube spacing but requires more horsepower; a lower average translates to narrower spacing using less horsepower. Using a wider delta t and boosting the inlet temperature should be considered to optimize designs.
Loop lengths primarily impact differential pressure a competent designer can play with temperatures, pipe diameter (fluid velocity), and tube patterns to overcome challenges of short or long loops. The caveat to this is when a circulator is improperly selected. A designer may calculate for a 20 degree delta t only to find out later the installed circulator was undersized and incapable of delivering the flow at design conditions ie: the head loss was too great resulting in a lower flow and thus a drop in the average fluid temperature or perhaps the right circulator was installed but the loop lengths were considerably longer than designed.
Different types of radiant floors require different designs.
An 8 thick concrete slab can easily use wider delta ts especially if the tubes are located towards the bottom of the slab. Tube patterns can equalize the surface temperature variations if necessary. A 1.25 light weight topping system on the other hand benefits from narrower delta ts to maintain constant floor temperatures. The floor covering will have a major impact on this. A tile floor resist heat transfer less than carpet so one could potentially notice a different surface temperature if the tubes are too wide or flow rate to slow as a result of designing for large delta ts.
Of course as Hot Rod has pointed out there is a difference between steady state vs dynamic state. Steady state is used in designing a system for maximum load but we know this rarely occurs because the load is always changing (dynamic). Cold weather start ups are a good example of where steady state design parameters are challenged.
In summary, the topic is really an intellectual exercise ( mental masturbation as per Bill Wrights commanding officer) designers select a delta t to determine the design flow understanding that at maximum load conditions the average fluid temperature will be maintained and deliver the required floor surface temperature with the appropriate tube spacing to maintain comfort conditions .should the load exceed the design parameters the average fluid temp will drop thus reducing the speed of transfer resulting in a drop in room temp if the load is less than design the average will rise thus improving the speed of transfer.
As Hot Rod pointed out .software is the best way to play this game but if you really want to do the hand calculations pick up a copy of ASHRAE 2000 HVAC Systems and Equipment manual, section S06 recommend you surround yourself with a calculator, pads of paper, pencil, sharpener, eraser, a large pitcher of water and bottle of Tylenol.
rb
ps: you'll need Adobe's free Acrobat Reader to open the attachments...let me know if you need help with this.
I'll be back on Thursday.0 -
Hey thanks! I have been playing around a little with the demo for Hydronics Design Studio. It's limited (Demo), but I can see how things change when I change things. The Btu/Hr. didn't change when I upsized the PEX size? The flow did. Strange, I thought it would...0 -
tube dia
The tube diameter contributes very little to the overall heat transfer i.e.; the surface area of a 1/2" pex vs. 3/4" is negligible and can be easily compensated with a slight rise in fluid temperature if necessary. What it does do is impact the fluid velocity and pressure differential so its more of a hydraulic issue than a heat transfer consideration. Its is why big does not mean more as is frequently misunderstood.
rb0
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