How overall heat transfer coefficient changes with water flow rate?

Answer 1

The overall heat transfer coefficient will increase slightly due to increased turbulence of the water. This is a small effect and the temperature difference across the exchanger of the water will be less. There will be more heat transferred across the exchanger both because of the lower water temperature difference and the slightly greater heat ransfer coefficient.

Answer 2

Those other web site references are a bit vague. The higher the velocity inside the tubes the better the overall heat transfer will be due to the breaking-down of the inside film coefficient caused by both the turbulence and the sweeping action. The sweeping action will retard the deposition of contaminants on the inside walls, usually referred to as fouling. Depending on the cleanness or turbidity of the raw water often used in surface condensers, a fouling factor is added to the theoretical overall coefficient. Heat exchanger design nomograms are sometimes available to evaluate the various relationships of tube diameter, thickness, number of passes, surface area, velocity and pressure drop. Of course most ot this is done with computer programs now, but they are probably based on the same old old nomograms. If the water is turbid, especially if sand is involved, there can be erosion at the entrance of the tubes and eventually failure can occur. Sometimes inserts have been employed to protect the tubes. Because of erosion there is a maximum velocity usually recommended which I do not remember. Because of rust, steel is not a popular tubing material. Stainless doesn't rust but the conductivity is poor and it is expensive. Copper and bronze are preferred, but now the costs are becoming very high. The outside condensing film cannot be avoided in condensers. The outside tube film in the shell side of evaporators is much better because of the turbulence of the boiling. In liquid-to liquid exchangers the outside film on the shell side is reducen by induced turbulence through the employment of baffels to direct the fluid in the shell to pass at multiple right angles across the tubes as much as is practical. When employing a glycol solution for sub-freezing evaporators, the effect on the film factors , the specific weights and the different specific heats of the various glucol solution concentrations is taken into account. The greater the temperature difference, the better the heat transfer and overall performance will be. A higher tube velocity will also improve the heat transfer to bring about a closer approach of the entering fluid temperature and the leaving fluid temperature on the other side , or in an evaporator, a better approach to the saturation temperature. This is desirable in refrigeration so the compressor can run at a higher suction pressure to save energy. Likewise, a foul of poorly turbulent condenser will cause higher discharge pressure in the condnser, likewise adding load to the compressor discharge. Here is where the efficiency comes into play. But again, there are other issues such as pressure drop, erosion, and manufacturing cost, which enter into the overall design.

Answer 3

increased water flow lowers the coefficient.