Handbook of air conditioning and refrigeration / Shan K

20.72 CHAPTER TWENTY
FIGURE 20.27 Air conditioning cycle—winter heating mode operation with space humidity control for a constant-volume single-zone
system.
1. Sensible heating process r-ru from return system heat gain
2. Mixing process ru-m-o of recirculated air and outdoor air
3. Sensible heating process m-ch at the heating coil
4. Steam injection or heated element humidifying process ch-h
5. Sensible heating process h-sf from supply fan power heat gain
6. Sensible cooling process sf-s from duct heat loss
7. Space conditioning process sr
The procedure for constructing this cycle is as follows:
1. Plot the state points of the conditioned space r and outdoor air o.
2. Draw the space conditioning line sr from point r with known sensible heat ratio SHRs. 3. From Eq. (20.87), calculate wr � ws. Since wr, Qrl, and the supply volume flow rate from summer
cooling mode calculations are known values, ws can be calculated. Point s must lie on line
sr, so the intersection of sr and ws line determines point s.
4. Draw a horizontal line r-ru from point r, representing the return system sensible heat gain. Plot
point ru from its known temperature rise.
5. Connect points ru and o. From the ratio of the volume flow rate of the outdoor air to the supply
air V˙ o / V˙ s,
point m can be found and located on line ru-o.
6. Draw horizontal line m-ch from point m. Draw also a horizontal line sf-s-h through point s. Temperature
difference Tsf � Th represents the temperature rise of the supply fan heat gain, and
Tsf � Ts is the supply duct heat loss. This defines points sf and h.