38 Humidification
RH control in cold stores
Humidity control is often essential to the smooth running of cold stores, but the type of humidity control varies from one application to another. John Barker of Humidity Solutions explains why some require dehumidification while for others humidification is the order of the day.
GIVEN THE RELATIONSHIP between temperature and relative humidity (RH), controlling RH in any application is a precise science. In cold stores, however, humidity control can be particularly challenging and certainly needs to be aligned to the purpose of each store. For instance, in some cases dehumidification will be required to prevent ice formation, while in others humidification may be a requirement to maintain the required conditions for the items being stored.
Dehumidification for cold storage When outside air enters a cold store, particularly in the summer months when humidity tends to be at its highest, it can lead to formation of ice and frost and a requirement for cyclical defrosting. Clearly this can be very disruptive, leading to interruptions in production processes with a negative impact on productivity. In most cases it will not be possible to completely prevent outside air from entering the cold store but it is possible to control the RH to keep processes running smoothly all year round.
In these situations conventional air dehumidifiers are not the most appropriate choice. Instead, desiccant rotor drying systems will provide a more cost-effective and reliable solution as they are able to reduce air humidity significantly, even at extremely low temperatures.
With a desiccant rotor drying system air in the cold store is passed through a silica rotor which absorbs water vapour from the air and moves it to another part of the equipment, known as the reactivation circuit. Warm air is then passed through
ACR News September 2014
this part of the rotor so that the moisture is exhausted to the atmosphere. Because this is a rotary system and the rotor turns continuously it is an ongoing process that maintains consistent control of RH levels. While the system is capable of
maintaining very good control of humidity in all cold store applications, the best results are often achieved by installing the units in antechambers with air locks, as this reduces the level of infiltration of humidity.
In such situations the design of the system is critical and correct sizing involves quantification of the infiltration of humidity and determination of the suitable dew point required. These systems can be retrofitted to existing cold stores and the humidity specialists will take readings of temperatures on the walls, floors and other structures where ice formation must be minimised. The dew point to be controlled must be slightly below this surface temperature.
Humidification for cold storage As noted earlier, there are also situations in cold stores where humidification is required. An example of this is when young trees and hedging plants need to be kept dormant until they’re ready for planting. This is the situation for J&A Growers, one of the UK’s leading wholesale growers of mostly native species of seedlings, trees and hedging for a wide range of purposes – including forest, hedgerow and roadside planting and landscaping.
Jamie Dewhurst of J&A Growers explains: “We maintain a temperature of 2°C at a relative humidity of around 95% to keep the plants dormant after they have been
harvested in the winter. This helps to extend the planting season and ensures that plants arrive on the planting site in optimal condition.”
As the refrigeration process dries the air in the cold store, humidification is essential to maintain the required 95% RH and avoid desiccating the dormant plants. However, it is equally important to avoid condensation in these low-temperature conditions, as this can lead to fungal attacks.
An older compressed air and water humidification system was failing regularly and proving expensive to run because of the use of compressed air and the cost of repairs. To overcome these issues a high- pressure adiabatic humidification system – designed, supplied and installed by Humidity Solutions – is helping to create the required conditions.
A key feature of the system is its use of high efficiency atomisers, which ensure a short evaporative distance and thereby avoids condensation.
A further benefit is that the adiabatic cooling effect of the system, when the water evaporates, helps to reduce the energy consumption of the refrigeration plant by providing 0.6kW of cooling per litre of water atomised.
These are just two examples of the wide range of humidity control requirements in cold store applications. Indeed, there may even be occasions where the requirement is for dehumidification in the summer and humidification in the winter to account for seasonal variation in RH. The key thing is to understand the exact requirements of each project and provide a solution that best meets its needs.
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