Dehumidification Systems

Transcription

1 Dehumidification Systems The purpose of this Bulletin is to give details of the preferred method of dehumidifying cold store loading dock pods, large loading dock areas and airlocks. 1.0 General Moisture infiltration is largely due to the higher air density in the cold store than outside therefore the cold air tends to fall out of the lower part of any door while warm air rushes in the upper part. Different air pressure on different sides of buildings also promotes air movement when there are multiple doors. No trailer door seal is completely effective as air will always escape past. Excessive moisture entering a cold store can cause a potential safety hazard with ice and snow forming around the doors and on the insulated panels and floors, thus making the floor slippery for its occupants. Dehumidification will reduce the effects of moisture ingress. There is also the added benefit of reducing energy consumption as the refrigerating plant is freezing less water (moisture) and defrosting less frequently. The number of defrosts would typically be less than one per day for a large -25 C room. A dehumidification system combined with strip curtains and inflatable door seals will reduce the moisture infiltration to a cold store by up to 80% when compared to the door seals alone. Fast roll or fast swing doors are often favoured over strip curtains. The power consumption of the dehumidifier is approximately 4kW per loading dock door. 1.1 Moisture Infiltration The design of a dehumidification system would take air in at ambient humidity from a loading dock say and remove as much moisture as possible prior to returning it to the loading dock. Table 1 below shows the moisture content in g/m 3 of air at its dew point. Most of the moisture has been removed from the air when it has been dehumidified to a dew point of 15 o C. It is uneconomical to dehumidify air below the cold store air temperature dew point, as the power consumption would be excessive. For UK applications a design ambient of 25 o C 60% RH is recommended. T o C g/m Table 1. Moisture content of air at its dew point Jan of 5

2 1.2 Desiccant Dehumidifier Operation The principle is shown in Figure 1. The moisture laden air from the dock pod passes through the drying section of the rotating desiccant drum which absorbs the moisture before the dry air is returned to the AIR FAN DRY PROCESS AIR SUPPLY DIRECTION OF ROTATION MOIST PROCESS AIR DESSICANT DRUM FILTER HEATER FILTER FRESH OUTSIDE AIR WET REACTIVATION AIR DISCHARGED OUTSIDE dock pod. The fresh outside air is heated and passed through the regenerating section of the desiccant drum. The moisture is rejected through the wet reactivation air discharge. Figure 1. Desiccant Dehumidifier Principle 2.0 Systems 2.1 Cold Store Loading Dock Pods The warm moist outside air will migrate into the colder cold store through the dock pod doors. As the warm moist air hits the cold air inside the cold store, it cools and the moisture falls out of the air forming ice inside the cold store. The re-circulation dehumidifier system reduces the level of moisture infiltration to the cold store door by dehumidifying the dock pod between strip curtains on the cold store opening and the outside door. The strip curtains reduce air infiltration by creating a barrier between the loading dock and cold store when the doors are open. The re-circulating dehumidified air in the loading dock tends to prevent air that has not been dehumidified entering the cold store. The relatively warm re-circulating air also tends to prevent the common problem of the strip curtains turning opaque. The positioning of the two long narrow air grilles on the air extract ducts below the ambient door and the position of the supply air grill near the cold store door combined with the given air flow rates specified below gives an effective dehumidification system. Figure 2 shows the layout of the dock pod, its ducting and air grilles. Jan of 5

3 The minimum recommended re-circulation rate of a dehumidifier for a single dock is 400m 3 /hr and for multi-dock pods is 320m 3 /hr/pod. The re-circulation rate is independent of loading dock size and volume. The dehumidifier chosen should be a maximum of a 2-pass system with a design air on temperature of 25 o C 60% humidity and an air off supply dew point of -15 o C. The method of specifying desiccant dehumidifiers must be clearly understood. The concept of a two-pass system to achieve a certain air off dew point means that air would have to go through the dehumidifier and back to the room then through the dehumidifier system again. Therefore if the moisture contents were 25g/m 3 on and 3.5g/m 3 off at the first pass, the second pass would give 3.5g/m 3 on and 1.36g/m 3 off. Obviously this doesn t happen in reality as the air from the first pass mixes with more moist air in the room, but it is an accepted way of specifying the units. The dehumidifier air off moisture content is lower if the air on is dryer, possibly due to continuous re-circulation or less air infiltration. The chosen dehumidifier air flow rate shown in Table 2 should suit the airflow recommendations above. Typically, the dock pod temperature will be maintained at around 15 o C when the doors are closed. The air ducting from the dehumidifier to the pod should be designed for an internal air temperature of 0 o C. The ducting shall be one layer of 25mm Koolduct on the supply air from the de-humidifier to the pod and on the re-circulation air inlet and outlet ducts. Two layers of 25 mm Koolduct shall be used on the return from the pod to the de-humidifier. An air velocity less than 5 m/s is recommended. Regeneration air ducts can be sized on 33% of the process air flow. 2.2 Large Loading Dock Areas The arrangement of doors is similar to cold store loading dock pods but without walls separating the pods. With this arrangement the same type of inlet and outlet grille set-up is required (normally with similar air re-circulation rates as the loading dock pod system) - see Table 2, however, a site survey will be required to assess the likely air intake through dock pods and other openings. If product is to be stored for long periods in the loading dock area, the supply air off the dehumidifier will require a cooling coil. Alternatively the loading docks own refrigerating plant will provide the cooling duty. 2.3 Air Locks Air locks are normally situated between chill and cold store areas to reduce moisture ingress when fork trucks are entering and leaving the cold store. A dehumidifier with a minimum recirculation rate of 400m 3 /hr is recommended. The moist return air intake grills should be located at the chill door and the dry process air grills should be located at the cold store door end of the air lock. It may be suitable to locate these dehumidifiers above the air lock with the fresh reactivation air being taken from the roof-void and the reactivation air discharged through the roof of the building. Typically, the air lock temperature will be maintained at around 15 o C when both doors are closed. 2.4 Alternative Dehumidification System Arrangements Jan of 5

4 Pressurisation using dehumidified air This system takes air from a chill store or heated air from a cold store mixed with ambient air and passes it through a dehumidifier directly into a cold store. This causes the cold store to pressurize. Results from sites show this method is not successful in large cold stores, as the quantity of dry air required is large. In addition, this dehumidification principle uses 3 to 6 times more energy than a re-circulation dehumidification system. Traditional refrigeration coil The conventional refrigeration coil system dehumidifies the air by passing it over a refrigeration coil to achieve the required dew-point temperature. The moisture condenses onto the coil forming ice. After a period of time the coil requires defrosting. To maintain dehumidification a second refrigeration coil is needed. A heater also is needed to heat the air prior to re-circulating it back into the room. This is a high capital cost solution. There are also high running costs associated with this type of system. For these reasons this type of system is much less attractive than the desiccant systems. Date Issue Originator 10/04/03 A DJC Jan of 5

5 MOIST PROCESS AIR EXTRACTED FROM DOCKING POD THROUGH LONG NARROW GRILLS SITUATED AS NEAR TO DOCKING POD OUTER DOOR AS POSSIBLE WET REACTIVATION AIR DISCHARGED OUTSIDE FRESH AIR INTAKE AND WET REACTIVATION AIR DISCHARGE SUITABLY POSITIONED TO AVOID RECIRCULATION FRESH OUTSIDE AIR DEHUMIDIFICATION DUCTING DOCKING POD WALL MOIST AIR EXTRACTION VENTS DOCKING POD DOOR DRY PROCESS AIR EJECTED INTO DOCKING POD AS CLOSE TO COLD STORE INSULATED DOOR AS POSSIBLE FIGURE 2 TYPICAL DEHUMIDIFICATION DUCTING LAYOUT SCRAP SECTION SHOWING POSITION OF MOIST AIR EXTRACTION VENTS DOCKING POD COLD STORE INSULATED DOOR (SHOWN IN OPEN POSITION) CURTAIN STRIP DOOR COLD STORE Jan of 5