Jump to Navigation

Enthalpy

Enthalpy controls are not well understood by many service technicians.  In order to understand the term “enthalpy,” the thermodynamics of air must be understood.

Common terms used in studying the properties of air are:

Dry Bulb Temperature:

The temperature reading on an ordinary thermometer.

Wet Bulb Temperature:

The temperature read on a thermometer whose bulb is encased in a wet wick with air blown over the wick at a minimum of 900 feet per minute.  Evaporation of the water from the wick causes the temperature to drop around the bulb.  When the temperature stops falling and stabilizes, that is the wet bulb temperature.  A sling psychrometer is an example of an instrument that can measure wet and dry bulb temperatures at the same time.

Relative Humidity:

The ratio of moisture in air compared to the maximum amount of moisture the air could hold at the same temperature and pressure.  A 35% RH means the air is holding 35% of the moisture it could hold at that temperature and pressure.

Saturation:

It is 100% RH.  No more moisture can be contained in the air.

Sensible Heat:

Heat that changes the dry bulb temperature of air without changing the moisture content.  Heat added by a coil is sensible heat.

Latent Heat:

The heat required to change water to a vapor without changing temperature or pressure.  When vaporizing water, latent heat is added to the air, and when vapor is condensed, latent heat is removed from the air.

Enthalpy (Often called “Total Heat”):

Enthalpy is the sum of sensible and latent heat in the air.  A psychometric chart can be used to ascertain enthalpy.

Figure 1 is a partial psychometric chart.  For clarity sake, some lines and scales found on the usual psychometric chart have been omitted.  Along the bottom of the chart are dry bulb temperatures.  The angled straight lines are wet bulb temperatures, and the curved and dotted lines are relative humidity.

Figure 1.

If one knows the dry and wet bulb temperatures, the RH can be found at the intersection of the two lines.  Examples:  The intersection of the 75°F dry bulb line and the 65°F wet bulb line is the 60% RH line.  The intersection of the 70°F dry bulb line and 70°F wet bulb line is 100% or saturation.  Extending the wet bulb line to the left shows the enthalpy of a pound of air.  In the above examples, the 75°F dry and 65°F wet bulb temperatures result in 30 BTU’s total heat or enthalpy in a pound of air.  70°F dry and 70°F wet bulb temperatures result in about 34 BTU/LB  of enthalpy.

Any intersection of dry and wet bulb readings can be extended to find BTU/LB of air by paralleling the intersection line with the wet bulb lines.

If the RH and dry bulb temperatures are known, an intersection of those readings marked on the psychometric chart will reveal the wet bulb temperature and BTU/LB enthalpy.  In fact, one can see that if any two figures are known, the other two figures can be found, as far as dry bulb temperature, wet bulb temperature, relative humidity, and enthalpy are concerned.  (Figure 1 is an enthalpy psychometric chart - not a complete psychometric chart.  Much more information can be found using a complete psychometric chart.)

Figure 2 is a chart that gives the properties of air and mixed in water vapor.  Enthalpy is shown only at saturation.

Figure 2.

Of what use is knowing what enthalpy is and how to find it?

One area where the knowledge can be applied concerns the thousands of economizer systems with sensible temperature changeover control.

Figure 3 illustrates a typical air handler.  The mixing section is where return air and outside air are combined.  There may not be a mixing section on some air handlers.  Some may be all return air or all outside air.

Figure 3.

The conditioning section usually has filters, heating and cooling coils, and may have a humidifier.

The fan section can consist of a supply fan only, as shown in Figure 3, or there may be a return or exhaust fan.  The supply fan shown is a pull-through fan because it is located on the outlet side of the conditioning section.  If the fan were on the inlet side, it would be a push-through fan.  The terminal section is all the components between the fan and the conditioned space or zoned spaces.  Many smaller systems may not have an exhaust damper.

Most air handlers are equipped with some kind of control system.  Often this is an “economizer.”  Simply put, an economizer uses outdoor air for free cooling, instead of the electrically fueled mechanical cooling system, as long as the outdoor air conditions are suitable for cooling.  Consequently, the economizer needs to be equipped with some kind of control to sense the properties of outdoor air and then make a decision whether to use outdoor air or not.

The control chosen most often to do this “changeover” measures only one property of the outdoor air:  temperature.

Changeover, based on temperature only, does not take into account humidity, and therefore, enthalpy of the air being brought in is not considered.

Figure 4 represents a typical air handler with an economizer system.  Thousands of systems like the one depicted by Figure 4 can be found in small office buildings, restaurants, motels, etc.

Figure 4.

 

Using round numbers and doing some raw figuring,

 an example can show how this air handler might perform

using only dry bulb temperature changeover: 

   Unoccupied setpoint of the room thermostat:  80° F.  Occupied setpoint:  70° F.

At the start of the occupied period, A1 is made to A2.  Assuming during the unoccupied period the space temperature rose to the 80°F setpoint, the setpoint suddenly is decreased 10°F to 70°F.  Y1 and Y2 will be powered. 

Let us say the outside air is just below 70°F and the humidity is 80% RH, a cool but humid day, which is not unusual spring or fall weather.  Relay R1 is energized closing contacts 1R1 and 2R1.

The discharge air sensor controls the damper actuator, whose setpoint is about 55°F. The outside air damper will be driven to its full open position; the return air damper will be closed, all in an attempt to use outside air for free cooling.  (The low ambient lockout control will be made R to Y.)

The air handled is rated 5000CFM.  A 5000-CFM air handler will be about 10 tons of mechanical cooling capacity, or 120,000 BTU/HR.

A cubic foot of standard air weighs .076 pounds.  This means the air handler is moving 22,800 pounds of air per hour.  (5000 x .076 x 60).

   In this scenario, all the air will be outside air. 

Using the psychometric chart, find the intersection of the 70°F dry bulb line and 80% RH line.  It is the 65°F wet bulb line.  Extending the wet bulb line to the left to find the enthalpy in a pound of air, we find there are 30 BTU’s in every pound of air.  22,800 pounds of air at 80°F and 80% RH contains 684,000 BTU’s. 

   Ideally, enough conditioning should be done to this air to achieve discharge air of 55°F at 50% RH. 

Using the psychometric chart, it is determined the air at 55°F and 50% RH has an enthalpy of about 18 BTU’s per pound.  The difference of 12 BTU/LB represents the amount of heat to be removed from the air to get to discharge air of 55°F at 50% RH.  22,800 x 12 = 273,600 BTU’s. 

The air handler’s maximum mechanical cooling system capacity is 120,000 BTU/HR.  If the air handler continuously brought in 100% outdoor air, discharge air of 55° at 50% RH could never be achieved. 

(In reality the discharge air temperature will slowly be reduced, and after some time that can only be empirically determined, the damper motor will begin to close the outside air damper and open the return air damper.  Eventually, as the return air is cooled, the dampers will find some position to maintain the discharge air temperature, and the space temperature will get to setpoint cycling the mechanical cooling (Y2).  Depending on conditions, even the economizer (Y1) may cycle.) 

The example is extreme, but illustrates the extra cooling capacity required to dehumidify cool, but humid air.

Modern electronic enthalpy controls are now available to replace temperature only changeover controls.  In humid climates, it is usually felt that about 50% of the cooling capacity of an air conditioning system is used to dehumidify the conditioned air (remove latent heat) before the sensible temperature begins to be reduced.  In most cases, cooling costs will be lowered when using enthalpy instead of dry bulb for changeover.

One of two strategies can be used when choosing enthalpy control:  

  •  Single enthalpy control uses one enthalpy sensor in the outdoor air. 
  •  Dual enthalpy control adds another sensor in the return air.

Using single enthalpy control on a call for cooling, the control compares the outdoor enthalpy to a pre-selected setpoint.  Setpoints vary, depending on climate, equipment, activity in the cooled space, etc.  The service technician can choose a setpoint that will result in less energy expended in dehumidifying the conditioned air, and consequently, lower cooling costs.

On Figure 1, observe the dry bulb 70°F line from the bottom of the psychometric chart to the 100% RH line.  If a dry bulb only changeover control set at 70°F is used on an economizer, everything to the left of the 70°F line represents outdoor air that will be used until the outdoor air exceeds 70° F.

Now, examine Figure 5.

Figure 5.

The curved lines A, B, C, and D represent setpoints of an enthalpy control.  The curved lines are changeover points just like the 70°F dry bulb line was the changeover point of a temperature only control, set at 70°F.  Since the enthalpy control is taking into account temperature and humidity, the lines curve.

To the left of the curved lines is usable outdoor air.  The control point is not fixed, but actually varies as the temperature and humidity of the outdoor air change.

Using the 5000-CFM air handler example, a comparison can be made between dry bulb changeover and enthalpy changeover.

Keeping the same outdoor air temperature of 70°F, but using an enthalpy control set to use curve B, from the psychometric chart Figure 5, outdoor air will be used up to about 50% RH.  At 70°F and 50% RH, there are 25 BTU’s in each pound of air.  The difference in enthalpy to reach 55°F at 50% RH is 7 Btu’s/LB.  22,800 LBS times 7 BTU’s equal 159,600 BTU’s.  A difference of 114,000 BTU’s that the mechanical cooling system doesn’t have to deal with as compared to a temperature only changeover control set at 70°F.

Granted, the examples are extreme, but they do illustrate how enthalpy control changeover can reduce operating costs of economizer systems in areas where humidity can be a factor.

Dual enthalpy control, often called differential enthalpy, uses the same outdoor air sensor as the single control system, but adds another enthalpy sensor in the return air.  The air with the lower enthalpy is brought into the conditioning section of the air handler.  This is a very efficient method of control.  The comparison is continuous, and again, savings can be verified using psychometric calculations.

Feature this resource?: 
No


Main menu 2