The requirements in industry for drying or dehumidification are numerous, and
there are many approaches that can provide economic solutions. In some cases,
the dehumidification heat pump can be the answer.
One example is in plastic products injection molding plants. One serious problem area in plastic molding has been the formation of condensate on the exterior of the molds, causing a summer slowdown in production. This moisture forms over the face of molds, resulting in defective products.
To better understand the difficulty, here's what happens in a production molding cycle. The casting molds have a number of cavities forming the specific item - Be it a bottle cap, surgical item, cutlery, or a child's toy. These molds have two parallel interior passages through which the equipment injects liquid plastic in excess of 500°F in one and a chilled glycol flow in the other. The plastic is then immediately hardened in the mold by the effect of chilled glycol at a temperature as low as 10°F. This process of forming a product can be as rapid as 6 seconds - from the time hot plastic is injected, set, ejected from the mold, and ready for the next cycle.
This rapid timing can be maintained as long as the equipment is located in a low humidity area, as would be encountered when outdoor temperatures are below about 50°F. However, as the temperature and humidity rise, condensate begins to form. And, even plant air conditioning does not eliminate the problem; sweating will still exist as long as the molds operate at that low set temperature.
To overcome the difficulty, manufacturers commonly raise the glycol temperature and thus the set temperature. This slows down production because the higher temperatures cannot harden the plastic as rapidly. The time cycle may double, and thus output is cut in half. This lost production is costly and presents a problem this industry has had difficulty overcoming.
One solution to this problem has been found, involving the use of a closed cycle dehumidification heat pump in conjunction with air-handling units containing a chilled water coil and reheat coil. In a typical system, the heat pump evaporator chills 48°F return water to 40°F, and this chilled water is used to cool plant air to about 51°F db and 50°F wb. The heat pump condenser is connected to the reheat coil to provide 115°F leaving air temperature from the air-handling unit. This dry air is discharged by means of a plenum placed directly over the mold area and prevents the formation of condensate.
The heat produced by the condenser will always exceed the requirements of the heating coils, and therefore a heat rejection device such as a cooling tower is required to dispose of the excess condenser output. This should be a closed-loop tower to avoid fouling the heating coils, which would be difficult and costly to clean. This would not involve a large tower, since disposal of this waste heat is at a temperature of about 130°F, unless this hot water can be used for other purposes, such as heating service hot water.
The advantage of using a heat pump instead of a chiller for dehumidification plus a hot water boiler for reheat, is obviously a lower total equipment installed cost and a lower operating cost.
