Salima Adjiri,
To clarify in case I do not entirely understand your question typical
absorption chillers have a refrigerant that is entirely internal to the
machine and then they have water as a working fluid in their cooling
liquid, chilled liquid and hot liquid loops. I am not certain whether
you are asking about how to change the assumed refrigerant in Type107 or
if you are intending to change the working fluid in the heat exchange
loops.

The type of refrigerant inside the machine gets wrapped up into the
performance data files that describe the machine's cooling capacity as a
function of the heat exchanger loop inlet conditions. I suppose that if
you had such data for a machine that used ammonia as its refrigerant
then you could reasonably model an ammonia based hot-water fired chiller
with it.

The difficulty with changing the working fluid in the cooling loop,
heating loop, or chilled loop to ammonia is that ammonia properties are
not very constant over the temperature range that you would likely be
working with. Type107 (and nearly all other TRNSYS components) assume
that specific heat and density of the working liquid is constant so you
have to be careful which fluids you specify.
kind regards,
David