For a given pressure, the addition of water to the ammonia will raise its temperature. At typical suction pressures, the addition of 10 percent water by weight will increase the evaporator temperature by about 4°F.
Stated another way, for a given evaporator temperature, the addition of water to the ammonia will require a lower suction pressure. For example, pure ammonia at -4° has a saturation pressure of 12.9 psig. An ammonia/
water solution of 10 percent water and 90 percent ammonia by weight at the same -4°, has a saturation pressure of 10.2 psig.