Transcript Document

Ins and Outs of Humidity
Basic to Advanced Investigations
What’s with the
wet bulb in here?
1 | Department (slide master)
Index
• Humidity defined
• Overview
• Units for measuring air
• Humidity’s impact
• Low Humidity
• High Humidity
• The Problem With Dry Bulb
• The Psychometric chart
• Overview
• Axis's and Values
• Plotting Basics
• Plotting Advanced
• Some More Fun
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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??
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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
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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
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Wet Bulb Temperature
Wet Sock
Moist environment, low evaporation rate
Dry environment, high evaporation rate
• 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
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Wet Bulb Temperature
Wet Sock
Moist environment, low evaporation rate
Dry environment, high evaporation rate
• 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’
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Impact of Humidity on Health
Why is this the case??
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Impact of Humidity on Health
• 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
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Impact of Humidity on Health
• 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
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Impact of Humidity on Health
• 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
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Impact of Humidity on Health
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.
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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?
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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
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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?
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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.
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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.
Fan
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Condenser
Evaporator
The Problem with Dry Bulb
Finding a Solution
• 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.
Fan
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Condenser
Evaporator
The Problem with Dry Bulb
Finding a Solution
• 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
Fresh Air
(Evaporator 2)
Exhaust Air
AC Coil
(Evaporator 1)
Return Air
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The Problem with Dry Bulb
Finding a Solution
• 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
Fresh Air
(Evaporator 2)
Exhaust Air
AC Coil
(Evaporator 1)
Return Air
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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)
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The Psychometric Chart
• 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
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The Psychometric Chart
75F DB 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
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The Psychometric Chart
Point of 80DB
and 70 WB
80F DB
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
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The Psychometric Chart
• 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.
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The Psychometric Chart
Point of 80DB
and 70 WB
75F DB
From the previous plot, we can see that the plotted point
contains ~ 92 grains of moisture
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The Psychometric Chart
A Sensible Process
Constant Grains
of Moisture
75F DB
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.
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The Psychometric Chart
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
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The Psychometric Chart
A Sensible Process
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.
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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
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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
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Questions????
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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.
Things We Can Measure
Enthalpy Example 1
Using a ruler, extend the line until it intersects
the enthalpy line and read the value*
Things We Can Measure
Enthalpy Example 2
Extend the db line until it intercepts the 65F
Wb line. Repeat the steps following from the
previous example
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.
Things We Can Measure
Relative Humidity (RH)
Follow the line from 70F Db at the bottom upward to where it crosses both
the 50% RH and the 100%RH (Saturated line.)
Things We Can Measure
Relative Humidity (RH)
110 grains
55 grains
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
Things We Can Measure
Dew Point
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.
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Things We Can Measure
Dew Point
Dew Point = 58F
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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
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Things We Can Measure
Specific Volume
Specific Volume Line
Specific Volume
= 13.5 ft³/lb.
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Things We Can Measure
Specific Volume
• The specific volume increases with an increase in DB Temp.
• This means the air is less dense at it heats up
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Things We Can Measure
Specific Volume
• The same occurs with an increase in moisture!
• Notice how the specific volume lines arc up and to the left?
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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.
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The ‘triangle’
Process Plotting
Getting Started
• We have already seen purely sensible and latent
process but in reality, especially for AC, the process
is a combination of the 2
Latent only drying
Sensible only cooling
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Process Plotting
Getting Started
• 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.
Latent only
humidification
Sensible only heating
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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!
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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
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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
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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
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Process Plotting
Doing another plot
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.
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Process Plotting
Doing another plot
Enthalpy=
32 btu/lb.
Enthalpy=
22 btu/lb.
Constant Enthalpy
43 grains
33 grains
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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
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Process Plotting
Apparatus Dew Point (ADP)
Entering 75 DB/ 67 WB
Leaving 60 DB/ 90%RH
ADP = 55F
ADP = 49F
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Entering 75 DB/ 67 WB
Leaving 56 DB/ 90%RH
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.
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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
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Process Plotting
Apparatus Dew Point (ADP)
ADP = 49F
The blue dot represents
the leaving condition with
a relatively high bypass.
The black dot represents
the leaving condition with
a lower bypass.
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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
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Process Plotting
Variable Speed Processes
• 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
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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
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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
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Load Changes and System Pressures
• 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
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Load Changes and System Pressures
• 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
R404A @ 20 psig
1 ft3 = 0.79 lbs.
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1 ft3
R404A @ 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
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Process Plotting
Variable Speed Processes
• 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
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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.
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Process Plotting
Some formulas
• 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
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Process Plotting
Some formulas
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
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Process Plotting
Some formulas
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.
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Things We Can Measure
Specific Volume
Enthalpy =
31.9 btu/lb.
Enthalpy =
27.5 btu/lb.
94 grains
66 grains
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Process Plotting
Some formulas
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
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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
=
= 7.3 lbs./hr
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400 x 28 x 60
13.2 x 7000