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# Refrigerator, do you know these basic refrigeration knowledge?

July 25, 2023

1. Temperature: Temperature is a measure of how hot or cold a substance is.

There are three commonly used degree-day (temperature scale): Celsius temperature, Fahrenheit temperature and absolute temperature.

Celsius temperature (t ,℃): the temperature we often use. The temperature measured with a centigrade thermometer.

Fahrenheit (F ,℉ ): The temperature commonly used in Europe and America.

Temperature conversion:F (℉) = 9/5 * t(℃) +32 (if the temperature in Celsius is known, find the temperature in Fahrenheit).

T (℃)= [F(℉)-32] * 5/9 (Find the temperature in Celsius when the temperature in Fahrenheit is known)

Absolute temperature scale (t, k): generally used in theoretical calculation.

Conversion between absolute temperature scale and centigrade temperature：T (k) = t (℃)+273 (the absolute temperature is obtained from the known temperature of Celsius)

2. Pressure (P): In refrigeration, pressure is the vertical force on the unit area, that is, pressure, which is usually measured by pressure gauges and manometers.

Common units of pressure are:

Mpa

Kpa

bar

kgf/cm2

atm

mmHg

Conversion relationship:

1Mpa=10bar=1000Kpa =7500.6 mmHg = 10.197 kgf/cm2

1atm=760mmHg=1.01326bar =0.101326Mpa

Generally used in engineering:

1bar = 0.1Mpa ≈1 kgf/cm2 ≈ 1atm = 760 mmHg

Several pressure representations:

Absolute pressure (Pj): In a container, the pressure generated by the thermal movement of molecules on the inner wall of the container. The pressure in the thermodynamic properties table of refrigerant is generally absolute pressure.

Gauge pressure (Pb): the pressure measured by a pressure gauge in the refrigeration system. Gauge pressure is the difference between the gas pressure in the container and atmospheric pressure. It is generally believed that the gauge pressure plus 1bar or 0.1Mpa is the absolute pressure.

Vacuum degree (H): When the gauge pressure is negative, take its absolute value and express it in vacuum degree.

3. Thermodynamic properties table of refrigerants: The thermodynamic properties table of refrigerants lists parameters such as temperature (saturation temperature) and pressure (saturation pressure) of refrigerants in saturated state. There is a one-to-one correspondence between the temperature and pressure of refrigerant in saturated state.

It is generally believed that the refrigerants in evaporator, condenser, gas-liquid separator and low-pressure circulating barrel are saturated. The vapor (liquid) in saturated state is called saturated vapor (liquid), and the corresponding temperature and pressure are called saturated temperature and pressure.

In the refrigeration system, for a refrigerant, its saturation temperature and saturation pressure are in one-to-one correspondence, and the higher the saturation temperature, the higher the saturation pressure.

The evaporation of the refrigerant in the evaporator and the condensation in the condenser are all carried out in a saturated state, so the evaporation temperature and the evaporation pressure, and the condensation temperature and the condensation pressure are also in one-to-one correspondence.

5. Superheated steam and supercooled liquid: under a certain pressure, the temperature of steam is higher than the saturation temperature under the corresponding pressure, which is called superheated steam. Under a certain pressure, the temperature of the liquid is lower than the saturation temperature under the corresponding pressure, which is called supercooled liquid.

The value of suction temperature exceeding saturation temperature is called suction superheat. The suction superheat is generally required to be controlled at 5 ~ 10℃.

The value of liquid temperature below saturation temperature is called liquid supercooling. Liquid supercooling generally occurs in the bottom of condenser, economizer and intercooler. Liquid supercooling in front of throttle valve is beneficial to improve refrigeration efficiency.

6, evaporation, suction, exhaust, condensation pressure and temperature

Evaporation pressure (temperature): the pressure (temperature) of refrigerant in the evaporator. Condensing pressure (temperature): the pressure (temperature) of the refrigerant in the condenser.

Suction pressure (temperature): the pressure (temperature) at the suction port of the compressor. Exhaust pressure (temperature): the pressure (temperature) at the outlet of the compressor.

7. Temperature difference: heat transfer temperature difference: refers to the temperature difference between the two fluids on both sides of the heat transfer wall. Temperature difference is the driving force of heat transfer.

For example: refrigerant and cooling water; Refrigerant and brine; There is temperature difference between the refrigerant and the air in the warehouse. Because of the existence of heat transfer temperature difference, the temperature of the cooled object is higher than the evaporation temperature; The condensation temperature is higher than the cooling medium temperature of the condenser.

8. Humidity: Humidity refers to the humidity of the air. Humidity is a factor affecting heat transfer.

Three representations of humidity:

Absolute humidity (z): the mass of water vapor per cubic meter of air. Moisture content (d): the amount of water vapor contained in one kilogram of dry air (g). Relative humidity (φ): indicates the degree to which the actual absolute humidity of air is close to the saturated absolute humidity. At a certain temperature, a certain amount of air can only hold a certain amount of water vapor. Beyond this limit, the excess water vapor will condense into fog, and this limited amount of water vapor is called saturated humidity. Under saturated humidity, there is corresponding saturated absolute humidity ZB, which changes with the change of air temperature. At a certain temperature, when the air humidity reaches saturation humidity, it is called saturated air, and it can't accept more water vapor; Air that can continue to receive a certain amount of water vapor is called unsaturated air. Relative humidity is the ratio of absolute humidity z of unsaturated air to absolute humidity ZB of saturated air. φ=Z/ZB ×100%。 It is used to reflect the degree to which the actual absolute humidity is close to the saturated absolute humidity.

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