A fluid enters a heat exchanger at 10 degC, exits at 60 degC and then enters another heat exchanger and leaves at 90 degC. Assuming there is no phase change, the mass remains the same and there are no thermal losses, if densities of the individual streams are given, is the mean density simply the sum of the three values divided by three?
Mean density, in my opinion, would be the fluid density evaluated at 50 C. This is the average of the fluid's inlet temperature and exit temperature. The average or mean density can be calculated as you suggest. It makes very little or no difference how the average fluid density is calculated.
Generally speaking, no. Do you mean the mean density over time? Then it depends on the time spent in each exchanger and in between. It also depends on whether this fluid's specific heat is as a function of time etc.
