HVAC 8: Psychrometrics 1: The Properties of Air

If the dry bulb temperature is 95F and the humidity is 100%, the wet bulb temperature will be ____________________ degrees and the dew point temperature will be ____________________ degrees.

95, 95

Why do relative humidity lines curve upwards as the dry bulb temperature increases?

As the dry bulb temperature of the air increases, it can hold more water vapor. Generally speaking, warmer air can support more moisture than cooler air can.

Define saturated air and explain why air in the saturated state cannot hold any additional moisture.

Saturated air contains 100% relative humidity and is always plotted on the leftmost upward curved scale on the psychrometric chart, representing both the 100% humidity line as well as the dew point scale. Since saturated air already contains 100% of the moisture that it is capable of holding, it is impossible for saturated air to hold any additional water vapor. This additional water vapor will condense on surfaces that are below the dew point temperature instead of being held by the saturated air.

VWhat is meant by standard air conditions?

Standard air conditions are used by manufacturers to rate the capacity of various pieces of equipment. These conditions assume that the specific volume of the air is 13.3 cu ft/lb of dry air and that the temperature of the air is 70F. It is also assumed that the air is at atmospheric pressure, namely 29.92 in Hg.

Explain where the Specific Volume scale is located on the psychrometric chart and what specific volume is commonly used to evaluate.

The specific volume scale is located along the bottom of the psychrometric chart and represents the volume that one pound of air occupies at the specified conditions. The units of specific volume are cubic feet per pound of dry air. Specific volume is commonly used by engineers to help evaluate fan performance and properly size motors for various refrigeration applications.

Briefly explain how the wet bulb thermometer on a sling psychrometer is able to record a lower temperature than the dry bulb thermometer.

The wet bulb thermometer is wrapped with a cloth sock that is saturated with distilled water at room temperature. When the sling psychrometer is rotated in the air, the moisture contained in the sock is permitted to evaporate. The rate of this evaporation is determined by the humidity of the surrounding air. If the humidity is low, the rate of evaporation is high, and vice versa. The evaporation of the water produces a cooling effect on the thermometer's bulb, thereby lowering its temperature below that of the dry bulb thermometer. If the humidity is 100%, none of the moisture from the sock can evaporate. This will cause the wet bulb and dry bulb thermometers to register the same temperatures, indicating a saturated air condition.

The psychrometric chart is

a. a graphical representation of the properties of air and how they relate to each other. c. used to determine various properties of air when at least two other properties are known. (Both A and C are correct)

The dew point temperature

a. is equal to the dry bulb temperature when there is 100% humidity. b. is equal to the wet bulb temperature when there is 100% humidity. BOTH A & B ARE CORRECT

The term that describes the amount of space occupied by a given amount of air is

a. specific volume.

Latent heat of vaporization is

a. the amount of heat that must be added to water in order to change it from a liquid to water vapor. b. the amount of heat that must be removed from saturated water vapor to condense it to a liquid. BOTH A & B ARE CORRECT

If air has a wet bulb temperature of 77F and a dry bulb temperature of 88F, what is the relative humidity of the air?

b. 60%.

Which of the following is a possible set of readings for wet and dry bulb temperatures?

b. Dry bulb temperature = 85F. Wet bulb temperature = 85F. c. Dry bulb temperature = 92F. Wet bulb temperature = 85F. BOTH B & C ARE CORRECT

In order for the sling psychrometer to take accurate readings

b. the sock on the wet bulb thermometer must be saturated with room temperature distilled water.

The gas law that states that the total pressure of a gaseous mixture is equal to the sum of the individual pressures i

c. Dalton's Law of Partial Pressures.

Which scale on the psychrometric chart is used to measure the total amount of heat that is either added or removed by an HVAC system?

c. Enthalpy scale.

Air is comprised of

c. dry air, water vapor and impurities.

If air has a wet bulb temperature of 65F and a specific volume of 13.5 cubic feet per pound, what is the dry bulb temperature of the air?

d. The dry bulb temperature cannot be determined form the information given.

The line representing the 100% humidity line on a psychrometric chart also represents the

d. dew point temperatures.

Moisture in the air is commonly called ____________________ and can be described as either relative or specific.

humidity

A _________________________ can be used by engineers and maintenance personnel to accurately measure the wet and dry bulb temperatures of the air.

sling psychrometer