![]() Did he (asdarw) specify that the substance was compressible? Otherwise, if you can assume an incompressible fluid, which based on his lack of supplying any other information on the problem, I was forced to make some kind of assumption, that's what I got.īut then again, I guess if it were an incompressible substance, it wouldn't be using an isentropic efficiency, which begs the question, is h an enthalpy? Quite small for an enthalpy. Without this information, you're just guessing.įor a compressible substance, yes. You need to know the inlet conditions of the steam entering the turbine (pressure and temperature) and exit conditions (pressure and temperature) in order to calculate the amount of work extracted. The change in energy is not determined solely by the difference in pressure the temperature of the steam is also changing as it expands through the machine. Turbines operate by extracting energy from the expansion of steam through the device. Let's make sure I understand the variables first: ![]() That being said, there is enough to calculate the delta P (but not a static inlet or outlet pressure as you pointed out) without having all the other information. Good old thermo.Īs for the second question, I'm thinking that perhaps the pressure is a change in pressure, or what I always call the delta P. I would have done the problem 100% the same. Good job on the first question, SteamKing. What's this equation, where is it derived from? It's in the answers for a question, the question asked to calculate the work done by the air in a cylinder as it expands out. The property info comes from the attached tables: The isentropic work (or more accurately, the power) is therefore W = m*(hi - ho) = 0.5 kg/s * (3423.1 - 2321.7) kJ/kg The enthalpy of this mixture can be calculated using the saturated liquid properties: S = 6.8826 kJ/kg*K = sf + x * sfg, where x is the quality of the vaporĭoing the algebra and solving for x = 0.870, or a vapor quality of 87% Then h = 3423.1 kJ/kg and s = 6.8826 kJ/kg*Kįor P = 30 kPa, we need to find the properties for s = 6.8826 kJ/kg*K, ![]() Since Mollier diagrams are hard to find on the web, you can also use steam tables. However, since you have specified a turbine operating isentropically, then using a Mollier diagram (a special diagram of enthalpy versus entropy), the state line of this turbine will be a single vertical line running from the inlet point down to the exhaust pressure of 30 kPa. Unless you know the enthalpy of the inlet and exhaust steam, yes, you need to refer to steam tables.įor your exhaust condition, knowing the pressure is often not enough information. ![]() Firstly, let's assume the expansion is isentropic.įrom this, how do we find the power generated and final steam quality?ĭo we look at steam tables, W = m(flow) (h_1 - h_2) - is that helpful? ![]()
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