FOR ALL PROBLEMS BELOW, start with the basic form of the energy balance as found

Question

FOR ALL PROBLEMS BELOW, start with the basic form of the energy balance as found in Eqn 5-33. Clearly list all assumptions made in deriving a simplified version of the energy balance appropriate to the device in question. You are taking a shower. Your shower head provides water at the EPA mandated maximum flow rate of 2.0 gpm at your preferred shower temperature of 112 F. The city water supply is at 58 F and your well insulated (220V) electric hot water heater provides hot water at 140 F. Determine the flow rates (in GPM) of cold and hot water required for your shower. Determine the rate of energy input required for the hot water heater Determine the electrical cost of a 20 minute shower using the most current PPL residential electric rate to compare. R-134 exits a throttling valve at 10 F with a quality of 35%. Assuming that it entered as a saturated liquid determine the following: The inlet temperature and pressure The pressure drop across the throttling valve

Explanation / Answer

2)10 F = -12.22 C

now at -12.22 C

now at -12.22 C and 35 % quality :

enthlapy = h2 = hf + 0.35 x hfg = 184.07 + 0.35 x 207.39

so, h2 = 256.66 KJ/Kg

a) now h2 = h1 = 256.66 KJ/Kg. as the processs is isenthalpic (throttlling)

so h2 = hf = 256.66 Kj/Kg

so temp = 40C (as h1 = 256..6 kJ/kG IS ENTHALPY OF SATURATED LIQUID AT 40 c) = 104 F

b) P2 at -12.22 C from R-134a table = 0.18524 Mpa

and P1 at 40 C, from R-134a rable = 1.0166 MPa

sp pressure drop = 1.0166 - 0.18524 = 0.83136 Mpa = 8.3136 bar

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