Measuring HVAC efficiency

Abstract

A method for measuring HVAC efficiency is disclosed which may be used to test various heat transfer or passive elements within a HVAC system to determine whether any element, such as a cooling coil, heating coil, filter or mixer, is operating at an acceptable efficiency when compared to design specifications or previously established standards. The method may also be used to easily determine volumetric flow rate of air through the system at various points.

Claims

1. A method for measuring HVAC efficiency for use with an HVAC system which includes at least one fan for moving air through the system, at least one heat transfer element which acts to effect heat transfer to or from the air within the system, wherein the direction which the fan forces the air through a duct within the HVAC system is considered downstream, the method comprising the steps of: (a) turning off all heat transfer elements; (b) after step (a), running the fan until the system reaches a steady state condition; (c) after step (b), measuring the enthalpy of the air within the HVAC system at a predetermined point in the duct downstream from all heat transfer elements; (d) after step (c), turning on a selected heat transfer element to be measured; (e) after step (d), running the HVAC system until it reaches steady state; (f) after step (e), measuring the heat transfer rate introduced to the HVAC system by the selected heat transfer element; (g) after step (f), measuring the enthalpy of the air within the HVAC system at the predetermined point; (h) using the measured heat transfer rate caused by the selected heat transfer element and the change in enthalpy of the system air to calculate the air flow within the HVAC system; (i) after step (h), adding a passive element to the system or modifying a preexisting passive element in the system, the passive element not acting to significantly effect heat transfer to or from the air within the system; (j) after step (i), running the HVAC system until it reaches a steady state; (k) after step (j), measuring the enthalpy of the air within the HVAC system at the predetermined point; (l) after step (i), measuring the heat transfer rate of the selected heat transfer element; (m) after step (l), determining the change in the heat transfer rate introduced to the HVAC system by the introduction or modification of the passive element performed by step (i); (n) after step (m), using the change in heat transfer rate and the change in enthalpy to calculate the air flow within the HVAC system; and (o) using the change in the heat transfer rate and the change in air flow between those values for the HVAC system obtained prior to step (i) and those values for the HVAC system obtained at steps (m) and (n) to determine the efficacy of the addition or modification of the passive element.

2. The method of claim 1 wherein the selected heat transfer element is a heating or cooling coil.

3. The method of claim 1, wherein the passive element is a filter or a mixer.

4. A method for measuring HVAC efficiency for use with an HVAC system which includes at least one fan for moving air through the system and at least one heat transfer element which acts to effect heat transfer to or from the air within the system, wherein the direction which the fan forces the air through a duct within the HVAC system is considered downstream, the method comprising the steps of: (a) turning off all heat transfer elements; (b) after step (a), running the fan until the system reaches a steady state condition; (c) after step (b), measuring the enthalpy of the air within the HVAC system at a predetermined point downstream from all heat transfer elements; (d) turning on a selected heat transfer element; (e) after step (d), running the HVAC system until it reaches steady state; (f) after step (e), measuring the heat transfer rate introduced to the HVAC system by the selected heat transfer element; (g) comparing the measured heat transfer rate of the selected heat transfer element to a rated heat transfer rate of the selected heat transfer element; (h) if the difference between the actual heat transfer rate of the selected heat transfer element and the rated heat transfer rate of the selected heat transfer element is more than a predetermined value, investigating the HVAC system to determine possible causes for the difference; (i), after step (h), inserting into the system a passive element or modifying a passive element already present in the system, the passive element not acting to significantly effect heat transfer to or from the air within the system; (j) after step (i), running the HVAC system until it reaches steady state; (k) after step (j), measuring the enthalpy of the air within the HVAC system at the predetermined point; (l) after step (j), measuring the heat transfer rate of the selected heat transfer element; (m) after step (l), determining the change in the heat transfer rate introduced to the HVAC system by the introduction or modification of the passive element performed by step (i); (n) using the change in the heat transfer rate determined at step (m) and the change in enthalpy to calculate the air flow within the HVAC system; and (o) using the change in the heat transfer rate and the change in air flow between those values calculated for the HVAC system before adding or modifying the passive element and after adding or modifying the passive element to determine the efficacy of the addition or modification of the passive element.

5. The method of claim 4, wherein the selected heat transfer element is a heating coil or a cooling coil.

6. The method of claim 4, wherein the passive element is a filter or a mixer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a typical pull-through HVAC system; and

(2) FIG. 2 is a schematic view of a typical push-through HVAC system.

DETAILED DESCRIPTION

(3) Referring to the drawings, in FIGS. 1 and 2 there are shown schematic views of pull-through and push-through HVAC systems. These Figures are intended to be representative of typical HVAC systems and to show the most commonly found elements within those systems. It is the method of analysis of such systems which is considered to be the invention rather than the systems themselves. The method of the instant invention analyzes and evaluates various elements of such systems including, for example, heating coils, cooling coils, and filters; but the method should work equally well with any element which causes a change in the enthalpy of the system air between the upstream side of the element and the downstream side of the element.

(4) Now referring to FIG. 1, a schematic view of a typical pull-through HVAC system is shown. A treatment node 2 is ordinarily located in an area which allows relatively easy access, but is usually totally enclosed inside a duct. A fan 4 is located near the downstream end of the treatment node 2 which pulls air through said treatment node 2 and pushes treated air through a series of ducts 6 throughout the building to locations where treated air is required. A cooling coil 8 is ordinarily located within said treatment node 2 just upstream of the fan 4. Most often a liquid, such as water, is chilled in some manner and directed through the cooling coil 8. The method of cooling the liquid is not considered important to the instant invention as long as there is a cooling coil 8 or any other element which performs the same function within said treatment node 2. A heating coil 10 is ordinarily located within said treatment node 2 just upstream of said cooling coil 8. Most often a liquid, such as water, is heated in some manner and directed through the heating coil 10. The method of heating the liquid is not considered important to the instant invention as long as there is a heating coil 10 or any other element which performs the same function within said treatment node 2 such as a heating coil.

(5) Still referring to FIG. 1, a filter bank 12 is ordinarily located within said treatment node 2 just upstream of said heating coil 10. One or more filters within the filter bank 12 act to remove particulates and other unwanted solid matter from the air stream. When filters become clogged, the efficiency of the HVAC system goes down because it becomes more difficult to pull air through the system. Many modern HVAC systems include an air blender 14 located within said treatment node 2 just upstream of said filter bank 12. Air entering a treatment node 2 often has separate layers which may have different properties including significant differences in temperature. Such layers make heat transfer at said heating coil 10 and said cooling coil 8 less efficient. An air blender 14 reduces such layering and makes the heat transfer significantly more efficient by making the air temperature more uniform across the duct 6.

(6) Still referring to FIG. 1, air returning to said treatment node 2 from the building through the ducts 6 enters an R/A duct 16 upstream of said treatment node 2. Quite often, depending upon the condition of the air inside and outside the building, some of this air is exhausted from the system as E/A 18 through an E/A damper 20. There may be an E/A fan (not shown) to accomplish this process. The remaining air (return air), R/A 22, passes through an R/A damper 24 and combines with outside air, O/A 26, which enters the system through an O/A damper 28. This mixture of R/A 22 and O/A 26 is pulled through said treatment node 2, treated, and pushed through said ducts 6.

(7) Referring now to FIG. 2, a schematic view of a typical push-through HVAC system is shown. Such systems are the same in all respects to the system shown in FIG. 1 except that said fan 4 is located upstream from said cooling coil 8 and said heating coil 10. In this Figure said fan 4 is located downstream from said filter bank 12 and said air blender 14, but it may also be located upstream from either of both of these elements as well.

(8) In both the pull-through and push-through HVAC systems disclosed in FIG. 1 and FIG. 2, various sensors and controls are nearly always included, but are not considered a part of the invention and are not shown. For example, the various dampers, said exhaust damper 20, said R/A damper 24, and said O/A damper 28, are usually opened or closed by a controller in response to feedback from sensors located within the system. In order for the method of measuring HVAC efficiency of the instant invention to work properly, it is necessary that an HVAC system to have some means of activating and deactivating the various elements such as said cooling coil 8 and said heating coil 10.

(9) Referring again to FIG. 1, the method of measuring HVAC efficiency of the instant invention is described here for said heating coil 10, but the same method may be used to determine the efficiency of various other elements of the system. All of the elements which have a direct and significant effect upon the enthalpy of the air flowing through the system are deactivated and the system run until the system reaches steady state. That is, the temperature of the air (both wet and dry bulb) at a particular point within the system remains relatively constant. The enthalpy (h.sub.off) is measured at point A which is downstream of the element to be tested. For best results, point A is the nearest point to said heating coil 10 where the temperature across a cross section of said duct 6 varies less than 0.5 degrees F. Said heating coil 10 is then activated and operated at full capacity until the system reaches steady state while the other elements remain deactivated. The same measurement repeated to obtain h.sub.on. The efficiency of said heating coil may be found by the following equation:
=((h.sub.onh.sub.off)/h.sub.off).Math.100%

(10) Rather than measuring the total air flow within the system using pitot tubes as it is conventionally measured; air flow may be determined using the delta enthalpy calculation described above along with well-known total heat (Q) formulas. If, for example, the heating coil were to be used for determining air flow, the volumetric flow rate of the water (V) through the coil would be measured as well as the change in temperature (7) between the water entering the coil and the water leaving the coil. The total heat (Q) can be determined using the following formula:

(11) $Q\left(\frac{\mathrm{BTU}}{\mathrm{hr}}\right)=V_{\mathrm{water}}\left(\frac{G}{\min }\right).Math.60\left(\frac{\min }{\mathrm{hr}}\right).Math.8.33\left(\frac{\mathrm{lb}}{G}\right).Math.1\left(\frac{\mathrm{BTU}}{\mathrm{lb}.Math.F.}\right).Math.T\left(F.\right)$

(12) G is gallons. The constants used in the above equation are for systems operating at sea level and adjustments will have to be made for systems operating significantly above sea level. Although water is used in the above example (8.33 lb/gal), the same equation (with different constants) may be used for other cooling or heating means. If, for instance, glycol were used instead of water, the similar known values for glycol would be used rather than that of water. The above formula is derived from Q=m*cp*T.

(13) This total heat value (Q) (from the above equation) can be used to determine air flow using the following formula which is well known in the art:
Q (BTU/hr)=V.sub.airflow (cf/min).Math.4.5 ((min.Math.lb)/(hr.Math.cf)).Math.h (BTU/lb)

(14) The value of Q in the above equation is the same as the value of the Q of the water as determined in the previous formula. The h value is the change in enthalpy of the air. The air flow may be found by solving the above equations for V.sub.airflow:
V.sub.airflow (cf/min)=(Q (BTU/hr))/(4.5 ((min.Math.lb)/(hr.Math.cf)).Math.h (BTU/lb))

(15) Again, the constant 4.5 will vary depending upon conditions (such as height above sea level and change in density) and units, but may be determined with relative ease.

(16) The above formulae are simply versions of the first law of thermodynamics with elements which are deemed to be insignificant to the instant invention left out of the equation. In its simplest form the above formula may be written as:
Q.sub.water=M.sub.air*h

(17) where Q equals the rate of change of energy, M equals the mass flow rate of air, and h equals the change in enthalpy with the element under consideration on and with the element under consideration off. Q.sub.air is equal to Q previously calculated for water in the heating coil. The equation may then be solved for M.sub.air which is the mass flow rate and the flow rate may be calculated using the known density.

(18) The following example should serve to make the method of the instant invention clear. Assume that an HVAC system has a hot water heating coil and that element is to be tested. All elements are turned off and the system run until equilibrium or steady state is reached. That is, the temperature of the air (both wet and dry bulb) within the system remains relatively constant. The enthalpy of the air is measured just downstream of said treatment node 2 at about point A on FIG. 1. Point A should be the point closest to the hot water heating coil where the temperature is relatively constant (within 0.5 degrees F.) across a cross section of said duct 6. Assume that this measurement shows an enthalpy (h) of 24 BTU/lb. Measurements are also taken to insure that the heating coil is off. That is, there is no hot water flow into the heating coil and there is no change in temperature across the coil. The heating system is then turned on to maximum capacity and the HVAC system again allowed to reach steady state. The flow of hot water through the heating coil is measured at 8.7 gallons per minute. The temperature of the input water is measured at 97 F. and the output water is measured at 81 F. The enthalpy at point A now measures 34. Note that said fan 4 should remain running both while all elements are turned off and while the hot water heating coil is turned on.

(19) These values may be plugged into the above formula as follows:
Q (BTU/hr)=8.7 (G/min).Math.60 (min/hr).Math.8.33 (lb/G).Math.1 (BTU/(lb.Math. F.)).Math.16( F.)

(20) or Q=69,572 BTU/hr.

(21) The above equation provides the total heat transferred from the water to the air flowing through the system. This total heat value (Q) can be used to determine air flow using the following formula:
Q (BTU/hr)=V.sub.airflow (cf/min).Math.4.5 ((min.Math.lb)/(hr.Math.cf)).Math.h (BTU/lb)

(22) The air flow may be found by solving the above equations for V.sub.airflow:
V.sub.Airflow (cf/min)=Q (BTU/hr)/(4.5 ((min.Math.lb)/(hr.Math.cf)).Math.h (BTU/lb))

(23) h is 3424 or 10. Q is 69,572 BTU/hr. Plugging these values into the above equation results in:
V.sub.Airflow (cf/min)=(69,572 ((BTU/hr))/(4.5 ((min.Math.lb)/(hr.Math.cf)).Math.10 (BTU/lb))

(24) or V.sub.Airflow=1546 cfm.

(25) Note that the air flow calculation is not strictly necessary for examining the heating coil efficiency, but is shown here as part of the example. Assume that the heating coil is rated to transfer 110,000 BTU/hr. By comparing this figure with the actual figure of 69,572 BTU/hr it is possible to determine that the heating coil is operating at about 62% design efficiency. Next the heating coil must be examined to determine which of several well-known problems are making the coil so inefficient. The coil may be covered with insulating dirt, the coil may be corroded, the boiler may not be providing sufficient hot water, or the air flowing through the heating coil may need to be blended prior to passing through the coil. In the case of this example, the coils were dirty and cleaning the coils changed the temperature of the water leaving the coil from 81 F. to 73 F. which changed Q to 104,400 which is much closer to the coil's rating. This same basic method may be used for other HVAC elements such as cooling coils. Note that the constant in the above formula, 4.5, may have to be changed using well established procedures depending upon height above sea level. Also note that the above calculations and formulae could also be used, using the same techniques to test whether a new HVAC systems meets specified operating parameters.

(26) The above described method of measuring efficiency applies to elements which are considered passive here as they do not, by themselves, significantly affect the enthalpy of the air in the system. For example, the inefficiency of the coil described above may be caused by inadequate mixing of the air just upstream of the coil. If so, the addition of an upstream air mixer may solve the problem and increase the efficiency of the coil. That is, the air mixer does not, by itself, significantly increase the enthalpy of the system, but it may increase the amount of heat transfer between the air within the system and the coil. As stated, the efficacy of the addition of an air mixer can be determined by using the above method, however, rather than cleaning the coil as described above, the air mixer is added. Otherwise the step is the same. Another example of such a passive element might be cleaning or changing an air filter.

(27) For heating elements the sensible heat formula may also be used to calculate the actual airflows by using the total heat from the water system in conjunction with the measured temperature found at the same location as the enthalpy described above. This well-known equation is as follows:
Q (BTU/hr)=V.sub.Airflow (cf/min).Math.1.08 ((BTU.Math.min)/(hr.Math.cf.Math. F.)).Math.T.sub.db( F.)

(28) Again the air flow is found by solving the above equation for V.sub.Airflow.
V.sub.Airflow (cf/min)=Q (BTU/hr)/((1.08 (BTU.Math.min)/(hr.Math.cf.Math. F.)).Math.T.sub.db( F.))

(29) The term T.sub.db is the dry bulb temperature taken at point A as described above.

(30) All instruments of the method for measuring HVAC efficiency are conventional and may be easily purchased at any of a number of scientific instrument providers.

(31) While preferred embodiments of this invention have been shown and described above, it will be apparent to those skilled in the art that various modifications may be made in these embodiments without departing from the spirit of the present invention. That is, the method could be used for a wide variety of purposes either in combination or separately.