The key factors and measurement of relative humidi

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Evaluation of relative humidity: key factors and their measurement

understanding the relationship between dry bulb temperature, wet bulb temperature, relative humidity and dew point temperature is very important for all aspects of air conditioning devices. These humidity issues play a particularly important role in building and material integrity, household health and comfort, and overall indoor air quality. Strangely, inappropriate humidity and temperature may cause residents to feel uncomfortable. Residents' complaints provide the HVAC contractor with the opportunity to find out the relevant adverse factors affecting the integrity of materials and indoor air quality in advance, including the breeding of microorganisms

when evaluating relative humidity, wet bulb temperature and dew point, HVAC technicians traditionally use a suspension hygrometer and a temperature humidity chart. Now they all use more accurate and convenient "humidity" meters, which can be measured even in narrow places where the suspension hygrometer cannot be used

standards adopted

many people believe that ansi/ashrae standards (humidity standards and IAQ standards) are adopted in the building specifications to prevent accidents. Since these two standards have been revised recently, the following description may help the monitor and contractor to correct their working methods to meet the new requirements

more comfortable

ansi/ashrae standard "thermal environmental conditions for human occupancy" sets the upper limit of absolute humidity (humidity ratio is 0.012 or 0.012 × 7000 = 84 moisture content/pound of dry air, also equal to the dew point (DP) of 62 ° f), above which most households will feel uncomfortable

since not all households will be satisfied with the same thermal state, especially at the same time, the standard attempts to determine a specification based on PMV (predictive mean vote - predicted average votes) with 80% satisfaction. From this, we can calculate the general thermal comfort dissatisfaction of 10% PPD (predicted percentage dissatisfied) - and the local ("my knees are cold") comfort dissatisfaction of 10% PPD

the standard lists six main factors affecting thermal comfort: metabolic rate, clothing insulation, air temperature, radiation temperature, air flow rate, humidity

the plastic granulator operation that understands these factors touches on an extremely wide range of areas, and the comprehensive impact helps engineers properly configure building systems


ansi/ashrae standard "ventilation for acceptable indoor air quality" stipulates that "the relative humidity of human living environment should be maintained between 30% and 60%, so as to minimize the breeding of allergic or pathogenic organisms."

revised ansi/ashrae standard 62 "Ventilation for acceptable indoor air quality" is more detailed. Now, the upper limit of relative humidity is based on the peak value. "Under the following two calculation parameters, the relative humidity of living space should be limited to a maximum of 65% or less:

1. Peak outdoor dew point calculation parameters and peak indoor latent heat load

2. The minimum space sensible heat ratio expected to occur and the outdoor parameters at this time.

good HVAC equipment selection practice usually suggests:

· winter parameters are 68 ° f to 70 ° F and 30% RH (relative humidity)


· summer parameters are 74 ° f to 76 ° F and 50% to 60% RH (relative humidity)

conditions are

· outdoor parameters are 97.5% in winter and 2.5% dry bulb temperature (DB)

in summer, which means that on average, 2.5% extreme temperature will exceed the capacity of the equipment. In these times, the capacity of the equipment is insufficient

since only 30% of the working time of comfortable refrigeration equipment occurs at 5% of the outdoor parameter dry bulb temperature, this is very important when selecting equipment. Although most commercial equipment is classified or has some form of capacity control, it is still very difficult to control the latent heat load in summer under partial load conditions

if the capacity of comfortable refrigeration equipment is large, there will be complaints related to humidity, and the problems will increase. Residential heat pumps should be selected according to cooling needs rather than heating needs, especially in areas where "smelly socks syndrome" is common and air conditioning equipment is located on low equipment floors


if you have enough knowledge and enough measurements, you can set the HVAC system in a suitable wet air state in summer and winter to prevent mold breeding. The conditions for mold breeding include spores deposited on the surface, a microenvironment with sufficient oxygen, suitable temperature, nutrition and water. Four of these conditions are available in almost every environment. The most controllable variable is moisture

the relative humidity higher than 60% can support the growth of mold on the hygroscopic (adsorbent) surface, and the hygroscopic surface at 80% relative humidity will promote the growth of mold. Almost all surfaces are or will become adsorbents, including painted surfaces, gypsum plaster walls, carpets, wall coverings, and masonry walls

masonry walls like bricks, cinders and concrete are excellent adsorbents, which can absorb a large amount of water and become a good place for mold reproduction. The water vapor pressure in the structural pores will be less than that in the surrounding environment, which will spread the water to the masonry pores. When the pores become moist, capillary effect will occur and fill the pores, thus providing an ideal breeding environment for fungal reproduction. This explains why some surfaces above the dew point also become very wet


the condition that condensation can occur on the surface is very obvious, so measures can be taken immediately. When the temperature of a surface is at or below the dew point temperature, condensation will form. Surfaces where condensation may occur include basement surfaces, surfaces of low equipment floors, cold water pipes, air conditioning equipment and pipe supports, as well as the interior of invisible skirt walls

since comfortable refrigeration equipment cannot control the humidity of the basement under the minimum heat absorption conditions, additional dehumidification equipment is generally required in the basement. Dealing with low equipment levels is particularly difficult and expensive. However, if there is no ponding or the ground is too wet (that is, it is assumed that the circulating air of the low equipment layer is not required for the equipment burning mineral fuels), the humidity of many low equipment layers can be controlled by using the damp proof layer up to the external ground for treatment and heat insulation, and integrating it into the controlled area as a whole, while adding other dehumidification methods. Water pipes can be insulated. The air-conditioning equipment and piping system must be sealed, airtight and insulated, and the damp proof layer must not be broken, especially outside the controlled environment. The piping system in all walls must be sealed to reduce the invisible moisture flow caused by the air pressure difference

in the refrigeration system, the relative humidity in the fresh air duct can be as high as 95% or higher, and the evaporator and condensate tray will be wet. Since moisture control is not feasible, it is very important to control spores and food (dust and suspended particles) in the air by using a good and very appropriate filtration system to control the breeding of mold. If the evaporator is resistant to ultraviolet (UV) radiation, the UVC "sterilization" lamp that can irradiate the entire evaporator surface can kill mold and microorganisms. The selected UVC lamp should not radiate ozone, which is a stimulant. Oversized equipment will shorten the working time and produce less condensate, which will actually promote the breeding of microorganisms on the condenser


Thermohygrometer like fluke 971 can complete the measurement from dry bulb temperature to relative humidity, and can calculate wet bulb temperature and dew point temperature. Wet aerodynamics points out that this is very critical for the evaluation and diagnosis of HVAC

· the wet bulb temperature is very close to the heat envelope, or the total heat in the air (dry bulb and wet bulb temperature). In the temperature humidity diagram, the wet bulb temperature line is close to and parallel to the heat function scale value. For the refrigeration system with fixed flow limiting valve measuring device, the return air wet bulb temperature line is mandatory in order to accurately inlet air

· the wet bulb temperature of fresh air and return air on both sides of the evaporator can be used to calculate the total temperature and humidity diagram or heat function table. Our experiments show that the refrigeration capacity, sensible and latent heat capacity, and s/t ratio

· use "CFM" × four point five × The total heat (QT = CFM × four point five × △h)。

· sensible cooling, latent cooling and s/t ratio can be obtained by drawing various states on the temperature humidity diagram or calculating from the temperature humidity diagram

· dew point is critical for both summer and winter assessments. The surface temperature of the pipe must be kept above the dew point to prevent condensation inside and outside the controlled area

· the indoor relative humidity in winter must be kept low enough to ensure that the surface temperature of interior walls and windows will not be close to the dew point. If condensation occurs on the surface of the window or wall, it is likely that condensation also occurs in the skirt wall

solve the problems related to comfort

if the equipment does not have sufficient capacity or is classified, when it operates based on the dry bulb temperature of the automatic regulating device, the operation time will be short under partial load conditions, that is, complaints related to humidity will occur. Shorter working hours mean less water is removed. Oversized equipment will only aggravate this situation and increase the chance of adverse conditions. Changing the fixed flow limiting device into a thermal expansion valve will ensure the maximum evaporation capacity under partial load conditions, and a larger coil surface can be used to remove moisture

most refrigeration equipment can withstand a 20% reduction in air volume. If the air volume of the evaporator is reduced from 400 cfm/ton to 325 cfm/ton, the temperature of the evaporator will be much lower than the dew point and more moisture will be removed from the air. This change will also reduce the pipe surface temperature and the temperature of the pass regulator towards the dew point temperature, affecting the air flow pattern in the occupied space

dehumidification device can reduce air volume under the condition of increasing humidity. Another optional method is to use the timed opening device to reduce the CFM value within minutes before the start of refrigeration, and then switch to the calculated CFM value until the end of the refrigeration cycle. Portable dehumidifiers can be placed in high humidity areas (such as basements) to reduce humidity, improve heat absorption, and force a longer cooling cycle. Please ensure ventilation in areas with intermittent high moisture absorption (such as bathrooms, kitchens, places for washing clothes) and outdoors (not attics or low equipment floors)

solve problems related to dew point and/or mold

pipes carrying wet and cold air in uncontrolled areas must be sealed with NFPA certified pipe adhesive. Any leakage of the pipe will lead to ineffective insulation at this point, and condensation is likely to occur. The insulation layer of the pipe wrapping shall not be flattened by the bracket. The bracket must be placed under the pipe wrapping. The thermal insulation layer of the pipe must not be broken, and the joint must be sealed

in an uncontrolled attic, the rise of the attic temperature may increase the heat absorption of the lower ceiling, but will reduce the chance of condensation in the pipe. In houses with newer building technologies, the temperature in the attic may be lower, but this will increase the chance of condensation on the surface of pipes or air conditioning devices. Seal the ventilation hole of the attic and add the projection lamp controlled by the humidifier to improve

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