Medium-temp Air Conditioning Equipment

Air conditioners can roughly be divided into two types: those offering comfort air conditioning for people, and those offering process air conditioning for industrial processes and objects besides people.
 
Process air conditioning is required at a wide range of temperature and humidity conditions for objects that require cooling, heating, humidifying, and dehumidifying. For example, to both maintain freshness and the work environment at food processing plants, medium-temperature air conditioning in the range of 10–25ºC, somewhat below the general air conditioning range, is used. Refrigeration and freezing units and specially designed refrigeration systems are also used in places where temperature control is required, such as places where food is temporarily stored.
 
There are many refrigeration and freezing control methods to control temperature, humidity, and other factors in the environment depending on the food item being stored.
 
Table 1 shows the typical temperature and humidity conditions for food storage.

This article provides an overview and describes important information about medium-temperature air conditioning for process cooling, using the example of brown rice.
 
Table 2 shows various medium-temperature air conditioning applications.
 
Rice Storage
 
The key point to pay attention to in rice storage is preventing mold from growing. Maintaining a ware-house storage temperature of 15°C or below is essential.
 
Temperature
 
The reasons for storing rice at 15ºC or below are to:
 
[1] Reduce the risk of mold, bacteria, and pests proliferating
[2] Avoid depletion of the nutrient reserves in rice through active respiration, causing the rice quality to change, and
[3] Prevent fat oxidation and carbohydrate breakdown.
 
Many types of mold grow and proliferate in the temperature range of 20–30ºC, but some types of mold can grow in the low-temperature range of 0–10ºC and at high temperatures of more than 60ºC.
 
For example, a type of mold that is a relative of aspergillus, which creates aflatoxins, can grow in the temperature range of 6–45ºC (optimum temperature of 32–33ºC) and create aflatoxins in the range of 12–40ºC (optimum temperature of 25–30ºC).
 
Relative Humidity
 
Mold growth is inhibited under dry conditions of low humidity. If the surrounding environment has relative humidity of 70% or less, many molds will have difficulty growing.
 
In general, when relative humidity reaches 70% or higher, condensation forms easily and mold and mites have an easier time proliferating.
 
Rice Moisture Content
 
Rice moisture can be managed during storage by controlling the rice warehouse temperature and relative humidity.
 
When brown rice is stored at 15ºC and the relative humidity of the storage facility is kept to 65%, rice moisture of about 15% can be maintained.
 
Storage Warehouses
 
Rice should be stored about 50–70 cm away from the walls of the warehouse, as this will help preserve its quality and facilitate inspections. Warehouses typically maintain a constant temperature using a low-or medium -temperature air conditioning system. When rice is stored in a low-temperature warehouse where the temperature is 15ºC or lower, it not only prevents rice’s freshness from deteriorating, but also inhibits the proliferation of mold and pests. These warehouses, however, must still take into account the following points.
 
• The temperature, humidity, and other warehouse conditions must be frequently checked, without relying only on automated management of the air conditioning system and other facilities.
 
• When storing rice at room temperature, the warehouse must still be inspected and cleaned at regular intervals.
 
Medium-temperature Air Conditioners
 
Medium-temperature air conditioners provide cooling in the range of 10–25ºC, which is lower than typical air conditioners designed for human comfort cooling. Medium-temperature air conditioners are used in various applications, including food processing, where product freshness must be preserved in a properly managed environment; server rooms, which require year-round cooling; precision equipment rooms and laboratories, which require a constant temperature to be maintained; and greenhouse cooling.
 
Major air conditioner manufacturers offer low-and medium-temperature packaged air conditioners (PACs). Air conditioners designed for human comfort cooling are relatively easy to purchase and install, while air conditioning and refrigeration systems for storage management of goods, particularly foodstuffs, are not necessarily simple. Manufacturers therefore offer packaged systems, which help simplify many of the challenges of air conditioning for storage management.

Up to the 1990s, low-and medium-temperature air conditioners offering cooling in the range of 15– 27ºC came in split types and modified packaged monobloc models. In 1997, Mitsubishi Heavy Industries (MHI) developed six cooling-only, ceiling-suspended low-and medium-temperature air conditioner models offering cooling in the range of 10– 25ºC. Ceiling- suspended PACs designed for food processing plants and food storage warehouses that handle fresh food at an indoor temperature of 10–25ºC were a groundbreaking product in Japan. MHI launched these six models together with three floor-standing cooling-only models providing indoor cooling in the range of 0–21ºC for a total of nine models. All of the models were air-cooled. With this product launch, MHI offered a complete lineup of air-cooled low-and medium-temperature PACs in ceiling-suspended, ceiling-embedded, ceiling-ducted, and floor-standing types, as well as water-cooled floor-standing models.
 
In the past, low-and medium-temperature air conditioners were intended for both humans and goods, so almost all of the air conditioners provided cooling in the range of 15–27ºC. These new air conditioners covered a wider temperature range of 0–27ºC and were able to meet needs for food storage and various food processing jobs. Subsequently, other leading manufacturers also created lineups of low- and medium-temperature PACs.
 
Low-temperature Rice Storage Warehouses
 
Daikin’s low-temperature brown rice storage warehouse series was popular with farmers. In June 2003, Daikin redesigned the models and increased the model variations to 10 types.
 
Low-temperature storage ware-houses for rice help prevent rice quality from deteriorating as well as damage from bugs, mice, and other pests. The low-temperature ware-houses are also versatile and can be used for storing other agricultural products besides rice. Daikin’s re-designed models offered a wider temperature control range as well as quieter operation. The new models also eliminated the need for drain-age pipe installation, which the former models required.
 
Daikin offers multipurpose and specialty brown rice models. The multipurpose models cover a wider temperature control range of 2–15ºC and have the following features.
 
For brown rice and other agricultural products: [1] warehouse temperature control range of 2–15ºC suitable for brown rice, fruits and vegetables, and other foodstuffs, [2] wide ambient temperature range of 0–43ºC, allowing installation in various locations without affecting the indoor temperature, and [3] drain-less operation not requiring drainage pipe installation.
 
According to Daikin, even at 2ºC operation, the condensate dis-charged from the unit is evaporated inside the unit, thus eliminating the need for drainage pipe installation.

An evaporation pan is placed in the unit, and the compressor discharge pipe is placed under the pan. High-temperature heat from the discharge pipe and warm airflow from the condenser evaporate the condensing water, eliminating the need for drainage pipe installation.

The specialty brown rice model stores brown rice at a temperature of 13ºC. It offers [1] temperature and humidity control specifically for brown rice storage (13ºC fixed temperature, relative humidity of around 70%), [2] standard types with basic features that forgo unnecessary functions, and [3] plug- and-play capability, with settings optimized for brown rice storage automatically set when the unit is plugged in.
 
Other air conditioner manufacturers including Mitsubishi Electric, Hitachi Appliances, and Toshiba Carrier offer low- and medium-temperature PACs. The operating temperature ranges vary slightly between manufacturers, with some models offering a minimum indoor air intake temperature of 10ºC.

Low air temperature discharge from the indoor unit is required to maintain a low storage room air temperature. To do this, the refrigerant evaporating temperature must be low. Various devices to modulate pressure and temperature are combined in the air conditioners for this reason, which increases product cost. But the higher price also offers packaged products the advantages of accuracy, safety, and durability over systems that are assembled on site, which can result in cost savings when all factors are considered.