Refrigeration Kylteknik

Transcription

1 Värme- och strömningsteknik Thermal and flow engineering Refrigeration Kylteknik Ron Zevenhoven Exam questions, max. points = = 30 All support material is allowed except for telecommunication devices m 3 (1000 kg) of water of 25 C is to be cooled quickly to 0 C and dry ice (solid CO 2 ) will be used for that. The CO 2 will evaporate and leave the water at 0 C. The water will stay in liquid state. a. How much dry ice (in kg), available at 1 atm, C is needed for this? (2 p.) Pressure is 1 atm; specific heat water c p = 4.19 kj/(kg K); dissolution of CO 2 in the water can be assumed to be negligible. The table gives thermodynamic data for superheated CO 2 at 1 atm. Enthalpy for dry ice at 1 atm, C is h = kj/kg. dm 3 /kg kj/kg kj/(kg K) b. In the above mentioned water, 200 kg of shrimp (specific heat c p = 3.62 kj/kg K) delivered at 15 C, would be cooled to and stored at 0 C. An engineer suggests to use the CO 2 vapour from the water tank (at 1 atm, 0 C) to pre cool the shrimp, using some sort of heat exchanger. The CO 2 would leave that heat exchanger at 10 C. For this case, again calculate how much dry ice (in kg), available at 1 atm, C is needed for this? (2 p.) 302. A natural gas (from Snøhvit, Norway) containing wt % of methane (C1) + ethane/ethylene (C2) + propane/propylene (C3) + nitrogen, respectively, is cooled, at ~ 60 bar from 12 C to 155 C and then throttled. A 3 stage mechanical vapour compression system is used with cooling against propane 3, ethylene 2 and methane 1, as in the process schematic shown below (next page). Also given below is an enthalpy pressure diagram for this natural gas, showing also the cooling down to 155 C, ~ 60 bar, followed by throttling. a. How much cooling in kj/kg NG is needed to bring it from 12 C to 155 C and how much of that is done in the methane cooling loop 1? (3 p.) continues on next page page 1 of 6

2 ** * The methane in cooling loop 1 leaves the last heat exchanger at 157 C, 1.4 bar (point * in the process scheme) as a saturated vapour. It is then compressed from 1.4 bar to 45 bar and cooled (using heat exchanger ** in the process scheme) to 20 C. b. How much heat will be transferred per kg methane in heat exchanger **? A pressureenthalpy diagram for methane (R50) is given below as well (page 3). Assume isentropic efficiency 90% for the compressor. (3 p.) page 2 of 6

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4 303. An air conditioning system is used to change the temperature, T, and relative humidity, RH = Φ, of a V = 45 m 3 /min air stream from T 1 =10 C, Φ = 30% to T 3 =25 C, Φ = 60%. For that, the air stream is first heated to 22 C in a heating section, followed by injection of hot steam in a humifier. See the process figure and the schematic trajectory in a psychrometric chart. Ambient pressure is 100 kpa. Determine a. the mass flow rate ṁ a (kg/s) of dry air, (2 p.) b. the heat input rate Q (kw) into the heating section, (4 p.) c. the mass flow rate ṁ w (kg/s) of the steam that is injected into the humidifier section (4 p.) Data can be found from the psychrometric chart above and/or the table below. Data water / water vapour: 5 C 10 C 15 C 20 C 25 C p sat Pa 872,1 1227,6 1705,1 2339,0 3169,0 h sat,liq kj/kg 20,98 42,01 62,99 83,96 104,89 h sat,vap kj/kg 2510,6 2519,8 2528,9 2538,1 2547,2 page 4 of 6

5 304. Liquefied air is produced using a series of five compressors with intercooling to the ambient surroundings (at temperature T 0 = 290 K), followed by throttling and gas/liquid separation, as shown in the process schematic and T,s diagram below. The five stage compression increases the pressure from 100 kpa (= p 0, ambient pressure) to 15 MPa. The compressors have an isentropic efficiency of 86%, with inlet temperature T 0 = 290 K, exit temperature T 1 = 400 K. A production rate of 100 kg/h liquid air is needed. Note the log p, h diagram for air given on the next page. a. For the compression stages, the optimum pressures are p 1 /p 0 = p 2 /p 1 = p 3 /p 2 = p 4 /p 3 = p 5 /p 4. Calculate p 1, p 2, p 3 and p 4 (in MPa) for p 0 = 100 kpa and p 5 = 15 MPa. (1p.) b. 1) Give the values for enthalpies h A, h B, h C, h Dl (for the liquid air) and h Dv (for the vapor air), 2) give temperature T at point C; 3) calculate the produced liquefied air amount γ (kg liquid air per kg air input) produced, and 4) calculate the necessary input stream of air (kg/h) (3p.) c. Plot the process in the log p, h diagram for air given on the next page and hand it in with your answers. (Extrapolate to obtain values for T 1 = 400 K). (2p.). d. Calculate the compressor power demand (as 5 the power demand for 1 compressor!) of this process plant (in kw). (3p.) e. If nitrogen would be used in this process, producing liquid N 2, which significant differences would you expect? (1p.) page 5 of 6

6 ÅA FNT KT VST Refrigeration exam Name: Matriculate number: page 6 of 6