1. Pressure (P): in refrigeration, the pressure is the vertical force on the unit area, that is, the pressure, usually measured by pressure gauge and pressure gauge.

Common units of pressure are:

MPa (MPA);

KPa (kPa);

Bar (bar);

Kgf / cm2 (square centimeter kilogram force);

ATM (standard atmospheric pressure);

MmHg (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 expressions:

Absolute pressure (PJ): the pressure on the inner wall of a vessel due to the movement of molecular heat. The pressure in the table of refrigerant thermodynamic properties is generally absolute pressure.

Gauge pressure (PB): the pressure measured by a pressure gauge in a 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, expressed by vacuum degree.

2. Temperature: temperature is a measure of how hot and cold a substance is.

There are three commonly used temperature units (temperature scale): centigrade temperature, Fahrenheit temperature and absolute temperature.

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

Fahrenheit temperature (F, f): the temperature commonly used in European and American countries.

Temperature conversion:

F (f) = 9 / 5 * t (℃) + 32

T (℃) = [f (f) - 32] * 5 / 9

Absolute temperature scale (T, ℃ K): generally used in theoretical calculation.

Conversion between absolute temperature scale and centigrade temperature:

T (o k) = t (℃) + 273

3. Refrigerant thermodynamic property table: the refrigerant thermodynamic property table lists the temperature (saturation temperature) and pressure (saturation pressure) of refrigerant in saturated state. The temperature and pressure of refrigerant in saturated state are one-to-one corresponding.

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

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

The evaporation of refrigerant in the evaporator and condensation in the condenser are in the saturated state, so the evaporation temperature and evaporation pressure, condensation temperature and condensation pressure are also one-to-one correspondence. The corresponding relationship can be found in the table of thermodynamic properties of refrigerants.

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

There are three ways to express humidity

Absolute humidity (z): the mass of water vapor per cubic meter of air.

Moisture content (d): moisture content (g) in one kilogram of dry air.

Relative humidity (φ): the degree to which the actual absolute humidity of the 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. If the limit is exceeded, the excess water vapor will condense into fog. This limited amount of water vapor is called saturation humidity. In 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, which can not accept more water vapor; the air that can continue to accept a certain amount of water vapor is called unsaturated air.

The ratio of absolute humidity Z of unsaturated air to absolute humidity ZB of saturated air is relative humidity. φ=Z/ZB ×100%。 It is used to reflect the degree that the actual absolute humidity is close to the saturated absolute humidity.