A heater is required to supply the domestic hot water DHW in a 50 unit apartment building where the DHW load is 150Lpd/unit, a total of 7500 Lpd. Most of the DHW is consumed between 6AM and 11PM, a period of 17 hours. Therefore the average consumption is 7.4 Lpm. The maximum DHW flow rate is 17 Lpm. Cold water is supplied by the city at 3 degrees C (Tc,i) and the DHW is supplied at 60 degrees C (Tc,o). The DHW is provided using heat from a boilder set at 75 degrees C (Th,i). In order to provide a highley effect heat exchanged adjust boiler water flow rate such that the boiler water exits at 65 degrees C (Th,o). One type of DHW is a shell and coil heat exchanger. Use this type with the DHW flowing inside three coils and the boiler water flowing on the shell side. The coils must be able to handle a flow of 17 lpm, with a resonable pressure drop. Use Type L 1/2" nominal (5/8" OD) copper water tube for the coils. When the length of the tube requried for the heat exchanger has been determined, check the pressure drop accross the coil. The tube must be wound into three coils, keeping in mind that the three helical coils are concentric and plumbed in parallel with one another. The minimum coil diameter is 5 times the tube outer diameter, otherwise the tube will kink when it is being wound into the helical coil. The pitch of the tubes in the coils, in the logitudinal and radial direction must be chosen by you. Consider the flow of boiler water over the coil on the shell-side. the shell is Ds mm diameter (inner). To improve the performance of the heat exchanger a Dcl mm cylinder (essentially a plug) is placed in the center of the innermost coil to prevent the flow of water up the center of the heat exchanger, thereby essentially bypassing the coil. You will ned to determin Ds and Dcl.
Calculate the required flow rate of the boiler water.
Calculated the shell-side and tube-side heat transfe coefficients.
Determine the total length of the 1/2" tube that is required to provide the DHW.
Determine the mean diameters of the three coils, and the length of tube in each coil.
The deisgn suggested above may not be optimal. Can you determin if there is a better configuration?