Ins and Outs of Humidity

Ins and Outs of Humidity
Basic to Advanced Investigations What’s with the wet bulb in here?

Index Humidity defined The Psychometric chart Overview
Units for measuring air Humidity’s impact Low Humidity High Humidity The Problem With Dry Bulb The Psychometric chart Axis's and Values Plotting Basics Plotting Advanced Some More Fun

Humidity Defined Overview
Humidity refers to moisture in air in vapor form This vapor contains energy equivalent to “steam” at the same temperature and pressure Can be a large portion of the heat energy contained in air In high concentrations it can impair the bodies capability to cool itself and promote the growth of illness causing pathogens* * Thought experiment: Why is salt sterile??

Humidity Defined Units for measuring
When we talk about temperature we need to define which temperature we are talking about To accurately specify a condition of air we really need to use both Dry Bulb and Wet Bulb Temperatures Definition of Dry Bulb and Wet Bulb temperatures Dry bulb (db) refers to the measurement of the physical energy of the air molecules Wet bulb (wb) refers to the temperature as affected by the rate of evaporation of water

Dry Bulb Temperature Dry bulb is the temperature that most people refer to when they refer to a temperature value of air, water etc. This temperature refers to the sensible only component of air and does not include the effects of water vapor

Wet Bulb Temperature Moist environment, low evaporation rate Dry environment, high evaporation rate Wet Sock Wet bulb temperature takes into account the moisture content of the air It does this by using the cooling effect of the evaporation of water into the air A wet sock is placed over the measuring section of a dry bulb thermometer

Wet Bulb Temperature Moist environment, low evaporation rate Dry environment, high evaporation rate Wet Sock Since evaporation requires heat, the process of evaporation cools the remaining water in the ‘sock’ below it’s dry bulb temperature The drier the air, the quicker the rate of evaporation and the lower the resulting temperature will be This difference between db and wb is called the ‘wb depression’

Impact of Humidity on Health
Why is this the case??

Your body uses the cooling effect of evaporation to remove heat from the bodies core To do this, sweat evaporating cools the skin and underlying tissues

Capillaries carry warm blood from the bodies core outwards near the surface Here the blood flows through the cooled area, gives up heat and returns to the core

Remember how high humidity lowers the rate of evaporation? By reducing evaporation, there is much less heat being removed from the outer layers of the body This means the blood that comes from the core cannot give up it’s heat so core body temperature rises

Cat + Dry Air + Styrofoam packing = In the winter time, the outside air is cold and contains very little moisture, even when it is saturated Air that enters a dwelling and warms up has the capability to hold far more moisture and will dry out any open source of moisture This includes the sinus areas which rely on mucus to prevent the entering of viruses etc.

Impact of Humidity on Air Pressure
Remember how people always say the air feels heavy when what they really means is it’s humid?? But why does it rain when there is a drop is atmospheric pressure? Can you spot the hurricane?

Impact of Humidity on Air Pressure
Due to the effect of water molecules having a high average space between them compared to air molecules, moist air is less dense than dry air This means that moist low pressure air is pushed upwards by cooler high pressure air moving in and can result severe thunderstorms

The Problem with Dry Bulb
It is a typical scenario. The room temperature rises and the t-stat brings on the AC. However there is more to conditioning of the air than just cooling it and the typical AC system also removes moisture from the air. However, what if the DB temperature is not high enough to force the T-stat to call for cooling? Or the latent load is just really high and there is a risk of moisture related problems occurring in the basement? Do you cool the whole house down to 64F just to remove enough moisture?

The Problem with Dry Bulb
The point that is classified as a “space-neutral” condition is 75°F DB and 50%RH. When the ambient condition is different from this condition, which is almost always, then the air inside will tend to deviate away from this point as well. Organizations like ASHRAE understand this dilemma all too well and as a result have recommended that geographic regions that have high latent to sensible ratios should have a separate dehumidification capability that does not necessarily rely on the air-conditioning operating. It is important to remember that indoor occupancy comfort is also paramount and making a space colder just to reduce humidity levels will only make matters worse.

The Problem with Dry Bulb Finding a Solution
So if supplemental dehumidification is the answer then, let’s look at how a dehumidifier actually operates. Most dehumidifiers have the same components that an AC unit has: evaporator, condenser, metering device and compressor. The main difference is in the air flows. While most AC units have separate air flows for the cooling and heat rejection sides, dehumidifiers have only one. Condenser Evaporator Fan

Having the condenser coil in series with the airstream leaving the evaporator allows the unit to operate as long as is required to remove humidity without overcooling the space. When you consider that AC was first implemented for humidity control rather than sensible cooling, you can see the irony in this. Condenser Evaporator Fan

Since fresh air requirements need t be met regardless of the outdoor air condition, manufacturers have developed flexible approaches to minimize the energy required to treat the air If the air is cool and dry, great just bring it on in. However, if it is cool but moist, a smart design will use just enough energy to treat the humidity issue and no more. Reheat Coil Dehumidification Coil (Evaporator 2) AC Coil (Evaporator 1) Fresh Air Exhaust Air Return Air

By utilizing an evaporator coil designed for latent heat removal and a low cfm per ton airflow, aggressive humidity removal can be achieved Combined with using the discharge gas to reheat the air, a mixture of dry treated air can be mixed in the required ratio to maintain a suitable indoor space condition Reheat Coil Dehumidification Coil (Evaporator 2) AC Coil (Evaporator 1) Fresh Air Exhaust Air Return Air

The Psychometric Chart
It looks intimidating, but when investigated step by step, it is not so bad! Above is a typical Psychometric chart used in the HVAC&R field (And was found on the web site noted above! J)

The psychometric chart for the air side processes is like the Mollier diagram for the refrigeration cycle processes It allows you to quantitatively plot what is actually happening when treating the air

75F DB Line 75F WB Line Lets look at the more pertinent values you can ascertain form the chart Here are the temperature scales for Wb and DB and 75F circled on both scales with the associated process lines

80F DB Point of 80DB and 70 WB To plot a point using DB and WB follow the process lines from each value until they intercept Her we show a plot for 80 DB and 70 WB

We often use grains for measuring water in psychometrics because the mass of water is very low and using grains instead of pounds allows us to use whole numbers On the right side is the Humidity ratio scale. This measures the weight of water in a pound of air In this case it is using grains of water as the unit There is 7000 grains to a lb.

75F DB Point of 80DB and 70 WB From the previous plot, we can see that the plotted point contains ~ 92 grains of moisture

The Psychometric Chart A Sensible Process
75F DB Constant Grains of Moisture A sensible process is one where the db temperature increases but the grains of moisture stay the same An electric heater is a good example of a sensible process.

90F DB Example: Air at 50F DB and 45F Wb is heated sensibly to 90F by an electric heater Plot the process on a psychometric chart

90F DB Find the point 50F DB and 45F WB Draw a line from this point following the grains of moisture line until it intercepts the vertical 90F Db line and stop. This is your air leaving point.

The Psychometric Chart A Latent Process
A latent process is a process where only the amount of moisture changes in the air but the dry bulb remains constant It follows the dry bulb line vertically up or down

The Psychometric Chart A Latent Process
Where as a purely sensible process is fairly common, a strictly latent process is very rare Almost all processes where there is a change in moisture also under go a change in sensible temperature as well

Questions????

Some More Things We Can Measure Enthalpy
Lets switch to a more detailed chart. On this chart the enthalpy scale is on the outside and bottom of the chart It shows how much heat energy in Btu/lb is present in the air.

Things We Can Measure Enthalpy
The total enthalpy in the air is a measure of the total amount of sensible and latent heat contained in a lb of air When the dry bulb or wet bulb temperature changes, it results in a change in the enthalpy of the air as well Example 1 : Plot the point of air at 75 F Db and 55F WB. Find the enthalpy at his point Example 2: If the moisture level of the air is increased until the WB temperature becomes 65F, what will the new enthalpy be?

Things We Can Measure Enthalpy Example 1
Plotted point at 75F DB and 55F WB.

23 btu/lb Using a ruler, extend the line until it intersects the enthalpy line and read the value* In reality, the Wb lines and the enthalpy line do not exactly run parallel. This becomes more pronounced at higher energy levels where both the DB and WB are high. At lower values though they are close enough to be accurate.

30 btu/lb 23 btu/lb Extend the db line until it intercepts the 65F Wb line. Repeat the steps following from the previous example In reality, the Wb lines and the enthalpy line do not exactly run parallel. This becomes more pronounced at higher energy levels where both the DB and WB are high. At lower values though they are close enough to be accurate. The enthalpy of the point in Ex. 2 is 30 btu/lb

Some More Things We Can Measure Relative Humidity
The lines that curve up to the right are the relative humidity (RH) lines Before we move forward, let’s define what RH actually means

Things We Can Measure Relative Humidity (RH)
Relative humidity is not an actual amount in terms of mass or energy It is a ratio of the amount of moisture that is in the air compared to how much the air can actually hold before it is saturated Air is said to be saturated if it is holding the maximum amount of moisture it can at it’s current temperature At 100% RH, the DB and WB temperatures will be the same!! Example : Air at 70F contains 55 grains of moisture per lb of air. If it is at 50% RH, how much moisture will the air hold at 100% RH (Saturated)? Simple logic tells us that at 50% RH, the air is only holding half of what it would hold at 100% RH. So doubling 55 grains = 110 grains. But to verify, lets look at the chart.

100% RH Line Notice how the wet bulb temperature on the 100% RH line is the same as the DB value. Follow the line from 70F Db at the bottom upward to where it crosses both the 50% RH and the 100%RH (Saturated line.)

110 grains 100% RH Line 55 grains Notice how the wet bulb temperature on the 100% RH line is the same as the DB value. If we follow the grains of moisture lines to the Humidity Ratio scale we can verify our values

Things We Can Measure Dew Point
We often hear the term dew point and even see a value for it when they are showing the weather statistics Dew point refers to the air temperature when air will be at saturation (100% RH) It is based on the fact that DB temperature affects the amount of moisture that the air can hold. Every day example : We see this effect when moist air rises and forms clouds. As the air rises it cools. When it cools down to the dew point temperature, the water condenses and forms clouds

Chart Example: Air that is 80F DB and 50% RH cools down on the side of a cold glass. If the glass is 40F, will the air be chilled down to it’s dew point and below? Assume the air can reach the same temperature as the glass Steps: On the psychometric chart, plot the point 80F DB and 50% RH Travel horizontally to the left until you hit the saturation line (100% RH). This is the dew point temperature. If this value is higher than 40F, than the air can become less than the dew point temperature and moisture will condense.

Dew Point = 58F

Things We Can Measure Specific Volume
Specific volume refers to the volume of air that is required to weigh 1 lb. It is the inverse of density which is lbs./ ft³ Both the sensible temperature and the amount of moisture affect the specific volume of the air and the psychometric chart is a great tool for demonstrating this. On the next slide, the specific volume lines are highlighted

Specific Volume Line Specific Volume = 13.5 ft³/lb.

The specific volume increases with an increase in DB Temp. This means the air is less dense at it heats up

The same occurs with an increase in moisture! Notice how the specific volume lines arc up and to the left?

Process Plotting Getting Started
We have already seen how to use 2 points such as DB/WB to find a point on the chart However, to be fully useful, we will need to plot actual processes as these are what are actually occurring when we treat the air Remember this shape, it will come in handy shortly. The ‘triangle’

We have already seen purely sensible and latent process but in reality, especially for AC, the process is a combination of the 2 Actual AC Process Latent only drying Sensible only cooling

You can do the same for heating and humidification This is where we heat the air sensibly and add moisture through a humidifier in the furnace. Actual Process Latent only humidification Sensible only heating

Process Plotting Now for some fun.
Let’s plot a typical air conditioning process where the air is cooled down through the evaporator to below it’s dew point Some of the moisture will condense out and drain away Thus it will end up being sensibly cooler and contain less moisture than when it entered. Now let’s apply this to the psychometric chart!

Process Plotting Doing a plot
While measuring the air return and supply from the indoor unit, we get the values below Entering 75 DB/ 67 WB Leaving 60 DB/ 58WB Calculate: Change in sensible heat (btu/lb) Change in latent heat (btu/lb) Grains of moisture removed (gr/lb) Plot these 2 points on the chart

Process Plotting Doing a plot
Total Heat Removed 31.6 btu/lb 29 btu/lb 25.1 btu/lb Latent Sensible Entering 75 DB/ 67 WB Leaving 60 DB/ 58WB Grains removed 86-79 = 17 grains

Process Plotting Doing another plot
Heating and Humidification Something to ponder: When an evaporative humidifier is placed in the outlet airstream of a furnace does the evaporation of the water change the total heat energy of the air?? In reality the change is very little, even if the water is cooler than the air. However the mix of sensible vs. latent does change. True or false? The addition of moisture (Latent heat) comes at the expense of sensible heat. True

Heating and Humidification Example to plot: Air enters a furnace at 70F DB and 30% RH. It leaves the heat exchanger at 110F. It then enters an evaporative humidifier which adds 10 grains of water per lb of air. Plot and calculate the total heat added to the air. The leaving air DB and WB.

Enthalpy= 32 btu/lb. Enthalpy= 22 btu/lb. Constant Enthalpy 43 grains 33 grains

Process Plotting ADP Apparatus Dew Point is the temperature of the coil required to achieve a leaving air condition that falls along the desired process line It is found by extending the process line until it intercepts the saturated (100% RH) line. The steeper the slope (Greater latent process) the lower the ADP required

Process Plotting Apparatus Dew Point (ADP)
Entering 75 DB/ 67 WB Leaving 60 DB/ 90%RH ADP = 55F Entering 75 DB/ 67 WB Leaving 56 DB/ 90%RH ADP = 49F

Process Plotting Coil Bypass
Because 100% of the air does not come into contact with the coil, the resulting leaving air is a mixture of air that has and has not been treated Typical coil bypass factors are .1 to .35 which means that 10 to 35% of the air does not come into contact with the coil The mixed air condition will fall along between the entering air point and the ADP point.

Process Plotting Factors Affecting Coil Bypass
Coils with a tighter fin spacing will have a lower bypass factor However, pressure drop across the coil also increases Another way to achieve a lower bypass is to reduce the velocity of the air At a lower velocity the air remains in contact with the coil for longer, greater mixing occurs and more moisture is removed

Process Plotting Apparatus Dew Point (ADP)
The blue dot represents the leaving condition with a relatively high bypass. ADP = 49F The black dot represents the leaving condition with a lower bypass.

Process Plotting Variable Speed Processes
A variable or multi speed fan is a great way to achieve a lower velocity for a dehumidification focused process while having the ability to run at a higher velocity for a more sensible process Care must be taken to protect the compressor against flood back Thought experiment: What can happen when the fan switches to a humidification process on a TXV metered evaporator and air flow suddenly drops

One of the issues with a fixed speed compressor is that when the load on the evaporator is high, such as during high latent loads, the TXV will open up and inject more refrigerant into the evaporator The result is a higher saturated evaporator temperature This is the result of the compressor needing higher density vapor to increase it’s pumping capacity

Equilibrium The main concept here is that the flow through the TXV must be balanced by the pumping capacity of the compressor This state can be referred to as ‘equilibrium’ In equilibrium, the pressures and temperatures do not change We see this when the load and ambient conditions are constant 65

Load Changes and System Pressures
If one of the parameters changes, say the air flowing over the evaporator becomes warmer, the equilibrium will be broken and the system conditions will begin to change In this case, if a TXV is used, the valve will begin to open and inject more refrigerant into the evaporator

This extra refrigerant flowing into the evaporator has to go somewhere and in this case it must be pumped thru the compressor However, in order for the compressor to move more mass of refrigerant, the density of the refrigerant must increase As a result, the pressure and temperatures in the evaporator will increase

For a certain volume, the higher the density of the vapor, the more mass it will contain This cylinder contains 50% more refrigerant 1 ft3 1 ft3 20 psig 1 ft3 = 0.79 lbs. 40 psig 1 ft3 = 1.21 lbs.

Process Plotting Variable Speed Process
This is representative of reality under high load ADP = 49F Here is the desired process

This issue can be over come by utilizing a variable speed compressor that can increase it’s pumping capacity to maintain a lower saturated temperature and thus maintaining effective dehumidification. In reality, both the air and refrigerant can be varied to match closely the desired conditions This allows for greater flexibility and optimal space treatment

Process Plotting Some formulas
The psychometric chart can provide us valuable information regarding the energy change per pound of air or how much moisture in grains of water was added or removed However, to be truly useful we need to convert these values into a format that is more suitable for using in an everyday application This is because systems are generally rated in Btu’s/hr, Tons of cooling or pounds of water per hour etc.

In psychometrics, we use certain formulas in conjunction with the chart. We use the values we obtain form the chart and plug them into the appropriate formula The sensible heat formula: Qs = 1.08 x ΔT x CFM Qs = Sensible heat in btu/hr 1.08 = A constant value ΔT = Temperature difference CFM =Air flow in cubic feet per minute

The Latent heat formula: Ql = 0.68 x ΔGr x CFM Ql = Latent heat in btu/hr 0.68 = A constant value ΔGr = Change in grains of moisture CFM =Air flow in cubic feet per minute The Total heat formula: Qt = 4.5 x Δh x CFM Qt = Total heat in btu/hr 4.5 = A constant value Δh = Change in enthalpy (btu/lb of air.) CFM =Air flow in cubic feet per minute

Let’s do an example 400 cfm of air enters a dehumidifier coil at 72F DB and 67F WB It leaves at 57DB and 56 WB. What is the latent heat removed in btu/lb? From the chart plot, obtain the latent energy enthalpy difference between the air entering the coil and air leaving.

Enthalpy = 31.9 btu/lb. Enthalpy = 27.5 btu/lb. 94 grains 66 grains

Results: Change in grains = 94 – 66 = 28 gr/lb The latent heat formula: Ql = .68 x Δgr x CFM Ql = .68 x 28 x 400 Ql = 7616 btu/hrs

Process Plotting One more for fun
How would you calculate lbs. per hours of water removed? A formula would need to convert cfm to lbs. per hours and grains to lbs. The result is useful for sizing a dehumidifier Lets use the results from the previous example Check this out: Lbs./hr H20 = cfm x ΔGr x 60 ft³/lb. x 7000 = 400 x 28 x 60 13.2 x 7000 = 7.3 lbs./hr

Ins and Outs of Humidity

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