Increasing supply and return temperatures

In summary: The equation does indeed tell you what your heat flow rate is, but I think the question you really are asking is how do you find the delta-T? The delta-T does depend on the functioning of the heat exchanger. There are several factors: -Approach temperature is the difference between the radiator (or whatever the heat exchanger is) temp and the air temp. Bigger approach temp=more heat transfer. -Flow rate: Over a short range and large approach temperature, flow rate and delta-T are directly proportional (so heat flow is constant), but if flow rate goes way down, delta-T widens a lot and the heat transfer goes down.
  • #1
TSN79
424
0
I'm trying to find the size of pipes that are to carry a certain amount of heat in them, 6000 W. This formula should give me the amount of water needed:

m=W/(4200*delta T)

Delta T is 20 degrees (the diff between supply and return), but what I don't get is that if I increase the supply and return temperatures to the extreme, shouldn't the radiator or whatever get increased effect? According to this formula it won't, since only the diff between the two temperatures is used. Can someone explain this to me??
 
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  • #2
The question is trying to get you to change the mass of transfer fluid rather than the temperature difference. I think you are missing the velocity limitation to determine the pipe size after finding the mass of transfer fluid.

Theoretically you could move any amount of mass through your pipe, to give that wattage and hold a 20 deg Delta T. Practically, you will be limited by the velocity and resistance in the pipe.
 
  • #3
TSN79 said:
Delta T is 20 degrees (the diff between supply and return), but what I don't get is that if I increase the supply and return temperatures to the extreme, shouldn't the radiator or whatever get increased effect? According to this formula it won't, since only the diff between the two temperatures is used. Can someone explain this to me??
You didn't provide any information about a radiator...

The equation does indeed tell you what your heat flow rate is, but I think the question you really are asking is how do you find the delta-T? The delta-T does depend on the functioning of the heat exchanger. There are several factors:

-Approach temperature is the difference between the radiator (or whatever the heat exchanger is) temp and the air temp. Bigger approach temp=more heat transfer.
-Flow rate: Over a short range and large approach temperature, flow rate and delta-T are directly proportional (so heat flow is constant), but if flow rate goes way down, delta-T widens a lot and the heat transfer goes down.

So... there are a lot of interdependent variables there. There are software packages available to calculate how that will all work out. Perhaps we could help more if we knew what exactly you are trying to do.

Oh... and if that's for a homework question, they may not want you to think about any of that stuff...
 
  • #4
Where did you get the 20 deg F(or C?) delta T? This number (20 deg F) is a convention used (and still used) to simplify pipe sizing. 20 deg F inserted in the calculation below simplifies down to a denominator of 9996 (which is rounded to 10000):

GPM = ((KWh*1000*3.412) / (DT * SpHtW * 60 * 8.33)

which will be one tenth of the numerator (the 3.412 converts the KW into MBH). Then the equation works out that the GPM is 1/10 that number MBH and from there it is an easy step to the pipe size, if you know the velocity you want to use. So designers would use the 20 deg DT so they could see the MBH and say the GPM is 1/10 of that.


In this case:

(6.0 * 1000 * 3.412)/ (20 *1 * 60 * 8.33) =
20472 mbh/10000 =
2.05 GPM

Is your delta T in F or C? What are the units for m that you are solving for? As you can see, I don't work in Metric very often.
 
Last edited:
  • #5
It should all be metric, and the Delta T is in Celsius.
 

Related to Increasing supply and return temperatures

1. Why is it important to increase supply and return temperatures?

Increasing supply and return temperatures is important for several reasons. Firstly, it can improve the efficiency of heating and cooling systems, as higher temperatures allow for better heat transfer. This can result in lower energy costs and reduced environmental impact. Additionally, increasing temperatures can also help to prevent issues such as condensation and freezing in the system.

2. What are the potential challenges of increasing supply and return temperatures?

There are a few potential challenges that may arise when increasing supply and return temperatures. One issue is the potential for increased wear and tear on equipment, as higher temperatures can put more strain on the system. Additionally, there may be concerns about safety and comfort, as higher temperatures can be uncomfortable for occupants and may pose a fire hazard if not properly managed.

3. How can supply and return temperatures be increased?

There are several ways to increase supply and return temperatures, depending on the specific system and needs. One method is to adjust the settings on the heating or cooling unit to increase the temperature output. Another option is to add insulation or make other improvements to the system to reduce heat loss. Additionally, utilizing alternative energy sources such as solar or geothermal can help to increase temperatures in an environmentally sustainable way.

4. Are there any potential drawbacks to increasing supply and return temperatures?

While there are potential benefits to increasing supply and return temperatures, there are also some potential drawbacks to consider. As mentioned earlier, higher temperatures can put more strain on equipment and may lead to increased maintenance costs. There may also be concerns about the environmental impact, as higher temperatures can contribute to greenhouse gas emissions and climate change.

5. How can supply and return temperatures be monitored and controlled?

It is important to regularly monitor and control supply and return temperatures to ensure that they are at optimal levels. This can be done through the use of temperature sensors and controls, which can provide real-time data and allow for adjustments to be made as needed. Additionally, regular maintenance and inspections can help to identify any issues or inefficiencies in the system that may be affecting temperatures.

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