Original sentence: A single-pressure segg absorption refrigerator uses three substances: ammonia, hydrogen gas, and water, whereas large industrial units generally use only two, a refrigerant such as ammonia, and an absorbent such as water (with an expansion valve and pump, not described here)
Edited sentence: A single-pressure absorption refrigerator uses three substances: ammonia, hydrogen gas, and water, whereas large industrial units generally use only two, a refrigerant such as ammonia, and an absorbent such as water (with an expansion valve and pump, not described here)