can someoen explain to me the effect or the meaning of U in heat exchangers? im using a cocurrent and countercurrent flow device. and i cant figure out whether the U is supposed to be different depending on the flow of the device. im assuming U is like a resistance, so the greater the value of U the more resistance, and the less heat transfer that occursthanks
the standard formula for thermal transfer using U(thermal coefficient) is Q=Au ( T1 - T2) Q is the absorption of heat in btu T1 is the temp differential internal T2 is the temp differential external A is the area of the material as an example lets do a problem Q= 200in sq (.19) (22degrees)=836 Q=200 in sq (.30) (22degrees)=1320 as you can see from the ex's that the higher the thermal transfer the greater the btu absorption. Respectfully the reverse or rather the loss of btu is the same. If you have a piece that is in operation measure the temp differential use a selected surface area and you can estimate the U value or you can look in the ASHREA manuals for a really long list. Since you mentioned it the relationship of U-value to the resistance is a relative matter the higher the number the greater the transfer,inversely 1/U = the R value which by practical example is more like the resistance to transfer....Well I hope I got in there somwhere with some help...Have a good one from the E
Define Overall Heat Transfer Coefficient
The heat transfer coefficient is actually different from resistance. Its dimensions are BTUs per hour per square foot per degree. It signifies the amount of heat that can be transferred for each square foot of heat transfer surface per degree of temperature difference between the two fluids. As far as co current very counter current flow is concerned, the U is the same. The difference comes from the formula for calculating the LMTD or log mean temperature difference between the two fluids as they enter and leave the exchanger. Check the sites below:
