Figure 2 is a graph of altitude conversion factor (FA) vs. These equations can only be used under standard air conditions.Īnother way to show how air density affects a coil’s required capacity is by comparing actual and standard cfm for the same mass flow rate (lb/hr).įigure 1 is a graph of temperature conversion factor (FT) vs. Therefore, there is the potential for misuse. Many people frequently use these equations without knowing how they were derived. Q = 1.08 x scfm x Delta T (sensible heat). Q = 4.5 x scfm x Delta hr (total heat) and The final form of the equation for standard air is: Q = 0.018 x scfm x Delta T (sensible heat)Ī quick check of the units reveals that the equations must be multiplied by the unit conversion of 60 min/hr in order to obtain the desired units for capacity of Btuh. Q = 0.075 x scfm x Delta hr (total heat) and For a sensible process, Delta hr = cp x Delta T where cp = specific heat of dry air = 0.24 Btu/lb/?F, where Delta T is the drybulb temperature difference between the entering and leaving air. This equation represents the required total capacity (total = latent + sensible) of the coil. Substituting this into the modified first equation yields q = 0.075 x scfm x Delta hr. As stated earlier, the density for standard air is 0.075 lb/cu ft. This equation can be further reduced for standard air. From this, it is clear that the required coil capacity is dependent upon the density of the air entering the coil. Therefore, the second equation can be rewritten as m = p x cfm, which when substituted into the first equation yields q = p x cfm x Delta hr. Volumetric flow rate has the units of cubic feet per minute or cfm. The product of the two yields the volume of air passing through the coil per unit of time or volumetric flow rate. Face velocity (V) has the units of feet per minute. The mass flow rate is equal to the density of air times the face area of the coil times the velocity of the air at the coil or face velocity:Īrea (A) has the units of square feet - fin height (inches) times the finned length (inches) divided by 144 sq in. Q = Heat transferred to or from the air (Btuh)ĭelta hr = Difference between the entering and leaving air enthalpy or total heat (Btu/lb) The required capacity of a coil can be calculated using the basic thermodynamic equation: Function of a Coil The function of a coil is to condition air such that when delivered will satisfy the space cooling or heating load. It’s a simple matter to demonstrate how the density of the air affects the calculated required capacity for a coil.įigure 1. At higher pressures the air will be denser, and at lower pressures the air will be less dense. Likewise, standard air pressure is defined as 29.92 in. Air that is cool will be more dense (heavier). Air that is warmer will be less dense or lighter. The density of the air depends upon both the temperature and pressure of the air.Īs stated earlier, standard air has a temperature of 70?. The two pieces of information programs use to calculate the actual density are the entering air temperature to the coil and the altitude at which the coil is operated. If the coil is run under actual conditions, programs need further information to calculate the density of the air. If the coil is run under standard air conditions (scfm), the programs automatically recognize that the density of the air is 0.075 lb/cu ft. In order to convert cfm to lb/hr, programs must know what the density, or the number of pounds contained in one cubic foot (lb/cu ft), of the air is.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |