What does the T-S diagram look like for an ammonia heat pump?

[swampwiz]

I'm a bit confused as to what the Temperature-Entropy diagram looks like for an ammonia heat pump. I understand perfectly how a freon type of refrigerant heat pump works: starting with working fluid at a state of a low-Temperature & high-Entropy gas (which is at low pressure), do work on the working fluid by compressing it, thereby resulting in an increase in Pressure & Temperature (and ideally no change in Entropy, although in "real life", there is always some small increase) such that the Temperature is higher than the high-Temperature ambient environment; then allow that working fluid to lose heat to that ambient environment, thereby resulting in a loss of Entropy such that the working fluid loses pressure until it hits the gas-liquid saturation state, and then continuing on by condensing the working fluid into a liquid; then allow the working fluid to depressurize, typically via a throttling device that achieves depressurization back to (or nearly back to) the initial pressure by friction (i.e., of the working fluid itself, or the friction with the containing vessel), which achieves a drop in Temperature (albeit that the friction actually causes a small rise in Entropy, but nowhere near the initial state) that is lower than the low-Temperature ambient environment; then allow the working fluid to gain heat from that ambient environment, thereby increasing the entropy to get back the initial state. The net effect is that the state of the working fluid moves in a counter-clockwise path such that there are 2 functional paths in which one of the paths has a lower Entropy than the other, and with the heat that is lost to the high-Temperature ambient environment is the area under the high-Temperature path, the heat that is gained from the low-Temperature ambient environment is the area under the low-Temperature path, and thus the difference of the two is the amount of work that must be done in the compression, and which also is the area bounded by the 2 paths.

What I don't understand is how to achieve this divergence of these 2 paths in the ammonia system that doesn't seem to use any pressurization. Certainly, the only way that an ammonia system could work is to somehow have a pair of state paths with some difference in between, but how is this done without pressurization?

 

[Navin A S]

Is it simply by the evaporative cooling of the ammonia, and thus a decrease in pressure that is sufficient enough to cause a drop in Temperature? Is there some other mechanism at work here? Any insight would be greatly appreciated.