Realize energy savings in cold store
The colder the climate, the more important it becomes to keep moisture under control, explains Kris van de Rijt, director of Air in Motion B.V. His company has developed solutions for managing the moisture load in cold stores and cold storages for many years.
We often come across situations in which customers have been ‘fighting’ for years to dehumidify via the evaporators. When moisture from a warmer climate enters the cold store through door motions, the only way to remove this moisture is via the outlet of the evaporators. During this process the moisture must precipitate on the cooling block of the evaporators, and then be discharged via the defrosting process as liquid water with a temperature of > 0ºC.
Often and in particular with small and medium-sized refrigeration installations, the defrosting process takes place by means of electric heating coils, which must heat the entire cooling block of the evaporator to above freezing point. During this defrosting process a lot of heat is lost to the freezing environment, which results into a rising temperature. In addition, the efficiency of the evaporator deteriorates considerably in case of ice formation on the cooling block in the evaporator. This efficiency can be reduced to 70% of the normal capacity. After all, less air can pass through the evaporator and the heat can no longer be transferred as efficiently to the cooling block of the evaporator due to the ice. Because of this, the energy costs, for maintaining the temperature, will increase.
Dehumidifier in front of or in the cold store
A common mistake is placing an additional desiccant dehumidifier in the cold store to solve this problem. Drying air in a cold store is an expensive affair. To further explain this it is important to understand how an desiccant dehumidifier functions. You can read it here.
Thanks to these unique properties of a desiccant dehumidifier, it is possible to dehumidify the air very efficiently even at low temperatures. However, the amount of energy, which is required to extract 1 kg water per hour from the air, increases considerably as the temperature further drops.
Comparison efficiency & energy costs
We take two identical desiccant dehumidifiers which both have a process are volume of 1.500 m3/hour:
1. Dehumidifier in cold store:
In a climate of -18ºC at 80% RH it is possible to extract approx. 1 kg/hour from the air. This means a dehumidification capacity of 24 kg/day. For this, a regeneration capacity of 13,5 kW is required. The efficiency is: 13.5 kW / kg water.
2. Dehumidifier in front of the cold store:
In a climate of 20ºC at 60% RH it is possible to extract approx. 10 kg/hour from the air. This means a dehumidification capacity of 240 kg/day. For this, a regeneration capacity of 13,5 kW is required. The efficiency is: 1.35 kW / kg water.
The difference (factor 10) in the energy requirement for dehumidification of 1 kg/hour is clear when a Dehumidifier is positioned in or just in front of the cold store. The question remains, however, how much moisture enters in practice at, for example, an average opening time of 1 hour per day?
| Comparison | Drying in front of the cold store | Drying in the cold store |
|---|---|---|
| Energy costs per year: | € 2,300.- | € 9,505.- |
| Percentage of energy costs per year: | 24% | 100% |
| Risk of defects in the event of power failure: | No | Yes |
| (Precious) loss of space in the cold store: | No | Yes, at least 1 pallet position |
| Adds (unwanted) heat to the freezing climate: | No | Yes |
| Creates underpressure * in the adjacent room: | No | Yes, estimated additional energy costs due to underpressure € 2,000.- / year |
*Underpressure is created when air is extracted from a room. When this room is cooled, warm and moist air will replace it in summer. When drying in the cold store, a cooled front space for sucking in the regeneration air flow is often used. This prevents internal moisture problems in the dehumidifier; however, it will cause an invisible extra heat and moisture in the front space. This invisible heat and moisture load costs several thousands of Euros per year in cooling capacity.
Moisture load in the cold store
If a passage of 3x3 meters (WxH) is open for a total of 1 hour during the workday (8 hours) to a climate of 20ºC at 60% RH, the moisture load per day will be approx. 200 kg.
With a correct separation technique at the entrance of the cold store in combination with a dehumidifier which is positioned in front of the cold store, a separation efficiency of >90% will take place. For more information, see our website about AFIM® Air Doors. The moisture load in the cold store is thus limited to only 20 kg per day. After working hours (approx. 16 hours per day) a dehumidifier in front of the cold store can automatically be switched off to save energy. Air In Motion is happy to advise you about the best solution for each specific situation.
However, if the dehumidifier is positioned in the cold store, it must remain activated 24/7 to avoid freezing symptoms. In addition the capacity of 24 kg/day is only a sloppy 10% of the moisture load during the workday.
Both the 10x more efficient dehumidification and the option to (automatically) switch off the dehumidifier outside working hours, makes dehumidifying in front of the cold store the only solution to avoid moisture load in the cold store.
Summary
We regularly face situations in which dehumidifiers are positioned in the cold store and dry the air of approx. -18ºC (or colder) in the cold store. In this article we show that this method of humidification is anything but profitable. The energy costs are more than 4x higher than necessary and with a worse result as a consequence.
Our solutions not only save energy, but also prevent slippery conditions on the work floor and dangerous situations around the passage to the cold store, says Van de Rijt. In the meantime, the solutions of Air in Motion are successfully applied in dozens of countries worldwide.
