FAQ 2018-04-26T11:06:14+00:00

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For the selection of a unit cooler, many aspects should be considered such as the size of the warehouse, the type of stored items, etc., but the first and foremost consideration should be the storage method. According to the storage method, it can be classified as follows.

Storage Method Room Temperature Purpose Distinctive Features
Refrigeration -20℃ ~ 10℃ Keep food or medicine fresh or cold. The 6.5-mm, 8.5-mm fin pitch and the propeller fan are mainly used.
Freezing -30℃ ~ -20℃ Keep frozen fish or meat fresh for storage. The 8.5-mm, 10-mm fin pitch and the axial fan are mainly used.
Congelation Below  -35℃ Quick freezing to maintain freshness of stored products. The 10-mm, 12-mm or longer fin pitch is mainly used. There is no restriction on the fan position in case of D, P, F type.
Because the capacity of Comp 1HP depends on the evaporating temperature, the Comp. capacity table must be considered to check the freezing capacity.
It sometimes occurs at a field that the Comp. 1HP is roughly calculated as the freezing capacity of 1RT, which is because the Comp. 1HP’s capacity at the evaporating temperature at 5 °C (evaporating temperature design reference for the air handling unit) is almost equal as the freezing capacity of 1RT.
Fin pitch Internal temperature Storage item
6.5mm Over -5℃ Low-temp storage other than workroom
6.5, 8.5mm 0 ~ -25℃
8.5, 10mm -25 ~ -35℃ Refrigerator, Freezer
10, 12mm Below -35℃ Freezing room

It is usually decided according to the internal temperature, the type of the stored product, and the expected frosting quantity. In general, it is better to set the criteria as shown in the above table.

Ton of refrigeration by global standard (1 CGS RT)

The refrigeration capacity required to make 1 ton of pure water at the temperature of 0℃ into the ice at the same temperature in 24 hours.
국제냉동톤(1 CGS RT)

Ton of refrigeration by the US (1 USRT)

The refrigeration capacity required to make 1 ton of pure water (2,000 Ib) at the temperature of 32°F into the ice at the same temperature in 24 hours.
미국 냉동톤(1 USRT)

과냉각 응축과정도

Definition

The subcooling refers to a refrigerant condensed and liquefied by the condenser of the freezer is cooled off again so that the temperature is below the boiling point for the pressure.

Degree of subcooling

When the refrigerant condensed in the condenser of the refrigeration cycle is cooled off to a degree that comes below the boiling point corresponding to the condensation pressure, this refers to a difference with this boiling point.

Purpose of subcooling

When the condensing temperature and the evaporating temperature are constant, the larger the subcooling degree is, the smaller the amount of flash gas generated decreases when it passes through the expansion valve. Therefore, the refrigeration capacity and the coefficient of performance increase.

  Compressor Condenser Action plan
Increased ambient temperature 1. Increased power required for compression
2. Rising temperature of discharge gas
(Deterioration and carbonization of oil due to cylinder overheating)
3. Increase in compression ratio
4. Decrease in volume efficiency due to decrease in the refrigerant circulation amount
5. Deterioration in refrigeration capacity
1. Increase in condensation temperature
2. Increase in condensation pressure
3. Increase in condensation load
Compressor and condenser should be selected and designed by fully considering the increase of the ambient temperature.
Decreased ambient temperature 1. Decrease in the required power for compression
2. Decrease in discharge gas temperature
3. Decrease in compression ratio
4. Increased refrigeration capacity
1. Decrease in condensation temperature
2. Decrease in condensation pressure
3. Decrease in condensation load
 

Cause

1. Excessive decrease of load in the evaporator
2. Excessive coil frosting in the evaporator
3. Excessive opening of expansion valve
4. Over-filling of refrigerant

Action plan

1. Install the liquid separator
2. Examine the defroster
3. Examine the expansion valve and temperature sensing tube
4. Choose the suitable orifice

Overview

In the evaporator that cools down the air, the air always holds moisture, thus, the moisture in the air becomes condensed and frozen to adhere to the surface of the cooling tube as frost. When the frost grows and its thickness increases, the amount of ventilation of the evaporator decreases and the heat transfer rate (K value) of the heat transfer tube reduces, disabling it to maintain the low temperature inside the system.
Defrosting is to remove this frost and recover the cooling function.

Influence that frosting has on the refrigeration device

(1) Temperature rise inside the refrigerator caused by worsened cooling capacity.
(2) Evaporating temperature and the vapor pressure decrease, therefore:
– Increase in compression ratio, decrease in volumetric efficiency, decrease in refrigeration capacity, decrease in performance factor → Increased power required per unit refrigeration capacity
– Rising temperature of discharge gas → Cylinder overheating, lubricant deterioration
(3) Increasing likelihood of liquid compression

Hot gas defrost

① The high-temperature and high-pressure gas coming from the compressor is put directly into the evaporator to heat up the evaporator coil to defrost.
– Large devices: Defrost with the latent heat of condensation (gas-liquid separation with the liquid separator at the evaporator outlet )
– Small devices: Defrost by sensible heat of hot gas (no phase change)
② Advantages
– No need to install an electric heater.
– Water piping is not required.
③ Disadvantages
– Defrosting is difficult to perform if the frost is too thick.

Water defrost

① Spray water on the evaporator coil to defrost.
– Recover the refrigerant in the evaporator, stop the fan and spray the water at the temperature of 10 ℃ to 20 ℃ on it.
– For this, the air outlet of the evaporator should be blocked.
② Advantages
– The most simple and standard method, and is widely used in unit cooler etc.
– Defrosting is available within a short period of time if applied with a high volume of water.
③ Disadvantages
– Water piping equipment required.
– Freeze prevention measures required for water supply pipes.
– Freezing of water may cause the fan to be stopped.
– There is a concern of possible splash of water on the coil when the fan starts to blow again after defrosting.
④ Method
– It is common to put the water to use in a tank (water tank for defrosting) and raise it to a certain temperature and recirculate it.

Electric defrost

① Install an electric heater at the proper position of the evaporator to defrost.
– Stop the fan and heat it when defrosting.
– Heater capacity is about 5kW per 1RT.
– Mainly used on small devices. In case of a large device, there is a problem with the manufacturing of a heater and repair for failure.
② Advantages
– Easy to operate and control an electric heater.
– It is possible to reduce heating unevenness by properly setting the heater installation method.
③ Disadvantages
– There is a need to install a circuit breaker, thermostat, etc. in the heater to beware of fire.
– There is a need to replace an electric heater after a certain period of use (heater becomes old after use).

Defrosting by antifreeze spray

① Method to prevent frosting by spraying antifreeze (brine) on the cooling pipe continuously.
– The frost is absorbed into the antifreeze as soon as it is formed, so the heat transfer surface always remains free from frost.
– Regenerate by heating the antifreeze with decreased concentration.
② Advantages
– There is no increase in the internal temperature caused by defrosting (because there is no need to stop the operation for defrosting).
– The heat transfer rate of the cooling pipe is large due to the wet surface effect on the non-frost and heat transfer surface. Therefore, the heat transfer surface can be made small (since the K value is good).
③ Disadvantages
– Expensive equipment cost (due to the use of regenerative anti-corrosive materials).
– Antifreeze should be selected in consideration of hygiene of stored products inside (to prevent damage to refrigerated products).
– Antifreeze which has high humidity at low temperature should be avoided.
④ Method
– There is a method of spraying antifreeze at all times or spraying it only in the case of defrosting.

The area of the outer surface where heat is transferred in the heat exchanger

Q (Kcal/h) = K * A * LMTD

  • Q= required heat amount by heat exchanger
  • K= heat transfer rate Kcal / m2 h ‘C
  • A= heat transfer area m2
LMTD; Logarithmic Mean Temperature Difference

  • LMTD = dt1 – dt2 / ln (dt1 / dt2 )
  • dt1 Temperature difference in the heat exchanger inlet side ‘C
  • dt2 Temperature difference in the heat exchanger outlet side ‘C
Heat transfer area m2 (A)= Q / K * LMTD.

The condensing capacity shall be calculated by adding the compressor power to the refrigeration capacity and then multiplying it with the correction value.

Q = (R + AW) * F

  • Q: Condensing capacity
  • R: Refrigeration capacity
  • AQ: Compressor power (kW * 860) (kcal/h)
  • F: Correction factor