värme- och strömningsteknik thermal and flow engineering
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Värme- och strömningsteknik Thermal and flow engineering Refrigeration 424159.0 Kylteknik Ron Zevenhoven Exam 24-3-2017
4 questions, max. points = 4 + 6 + 10 + 10 = 30
All support material is allowed except for telecommunication devices.
301. 1 m3 (1000 kg) of water of 25°C is to be cooled quickly to 0°C
and dry ice (solid CO2) will be used for that. The CO2 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, ‐78.69 °C is
needed for this? (2 p.)
Pressure is 1 atm; specific heat water cp = 4.19 kJ/(kg∙K);
dissolution of CO2 in the water can be assumed to be negligible.
The table gives thermodynamic data for superheated CO2 at 1
atm. Enthalpy for dry ice at 1 atm, ‐78.69°C is h = ‐259.51 kJ/kg.
b. In the above‐mentioned water, 200 kg of shrimp (specific
heat cp = 3.62 kJ/kg∙K) delivered at 15°C, would be cooled to and stored at 0°C. An
engineer suggests to use the CO2 vapour from the water tank (at 1 atm, 0°C) to pre‐cool
the shrimp, using some sort of heat exchanger. The CO2 would leave that heat exchanger
at 10°C. For this case, again calculate how much dry ice (in kg), available at 1 atm, ‐78.69
°C is needed for this? (2 p.)
302. A natural gas (from Snøhvit, Norway) containing 89.7 + 5.5 + 1.8 + 2.8 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
dm3/kg kJ/kg kJ/(kg·K)
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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 pressure‐
enthalpy diagram for methane (R50) is given below as well (page 3). Assume isentropic
efficiency 90% for the compressor. (3 p.)
1
*
2
3
**
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303. An air conditioning system is used to change the
temperature, T, and relative humidity, RH = Φ , of a V
= 45 m3/min air stream from T1=10°C, Φ = 30% to
T3=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 ṁa (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 ṁw (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
psat Pa 872,1 1227,6 1705,1 2339,0 3169,0
hsat,liq kJ/kg 20,98 42,01 62,99 83,96 104,89
hsat,vap kJ/kg 2510,6 2519,8 2528,9 2538,1 2547,2
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304. Liquefied air is produced using a series of five compressors with intercooling to the ambient surroundings (at temperature T0 = 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 (= p0, ambient pressure) to 15 MPa. The compressors have an isentropic efficiency of 86%, with inlet temperature T0 = 290 K, exit temperature T1 = 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 p1/p0 = p2/p1 = p3/p2 = p4/p3 = p5/p4.
Calculate p1, p2, p3 and p4 (in MPa) for p0 = 100 kPa and p5 = 15 MPa. (1p.)
b. 1) Give the values for enthalpies hA, hB, hC, hDl (for the liquid air) and hDv (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 T1 = 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 N2, which significant differences would
you expect? (1p.)