Quiz4s - relative humidity We begin by tracing the process...

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Fall 2007 1 ME 416 Computer Assisted Design of Thermal Systems Quiz #4 Solution A cooling coil is used to take air at 0.06 kg of dry air per second, 31 ° C, and 70% relative humidity and cool it to 18 ° C. What are the heat transfer rate required (in kJ) and the amount of water that must be drained away (in gm/s)? You may assume that the liquid water leaving has an enthalpy of 85 kJ/kg of liquid water. Solution: We begin by identifying whether or not we have condensation occurring. Going to the psychrometric chart at T DB =31 ° C and φ =70%, we find ϖ in = 20 gm/kg of air, T DP,in = 25 ° C and h av,in = 82 kJ/kg of air Since T out < T DP,in we will have condensation occurring. Then Water Conservation: = m m in air out , out ϖ + ϖ & & l Energy Conservation: ( 29 h - + h + m q = h out , out in out av, a out in av, l & & ϖ ϖ Since we have condensation we know that at our outlet state we must have 100%
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Unformatted text preview: relative humidity. We begin by tracing the process on the psychrometric chart. Going to the psychrometric chart at T DB =18 ° C and φ = 100%, we find ϖ out = 13 gm/kg of air and h av,out = 51.5 kJ/kg of air Then solving our water conservation we have 7 13 20 m m out in air out , =-= ϖ-ϖ = & & l gm/(kg of dry air) Using the mass flow rate of air we find that we will need to drain 0.42 gm of liquid water per second. For the heat transfer rate, we calculate from the energy equation air) of kJ/(kg/s 91 . 29 ) 85 )( 007 . ( 5 . 51 82 h m m h h m Q out , air out , out , av out , av air out =--= --= l l & & & & The cooling coils must be able to remove 1.8 kW. Fall 2007 2...
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