P-Traps were Promoted by Thomas Crapper for Toilets, not HVAC Systems or Critical Facilities Nicholas H. Des Champs, Ph.D., PE, ASHRAE Fellow
When using a toilet to get rid of human waste or a sink to wash vegetables, the discarded product ends up in the same sewer line prior to its destination, the septic tank or sewage treatment plant. Obviously, you would not want the gases from the toilets to penetrate the kitchen where food is being prepared. An airtight seal is required between the appliances and the sewer line. Since the end of the 19th century, P-Traps have been used successfully for this purpose, for each incident enough water flows through the trap to move the waste from the source to the sewer while leaving clean water within the trap as a seal against air flow. There are some problems associated with the P-Trap when used in disposal systems, such as dry-out or freezing, but the benefits outweigh them.
Since P-Traps seal against air leakage, they were the natural solution to prevent air leaking at the condensate drains of early 1900’s air conditioning units. As air conditioning caught on in the 50s, PTraps were the standard that every contractor field designed, and installed, even if it was unknown to them whether the drain pan was at negative or positive pressure, or even at what level of pressure. It is still that way today. Air conditioning units are shipped from the factory to the job site without a drain trap. The AC equipment it set on the slab, floor, or curb and it’s the mechanical contractor’s job to install a P-Trap. Presently, most plumbing codes call for the use of a P-Trap on all appliances that are connected to a sanitary sewer. But the only code for traps on HVAC units is to install a trap in accordance with the unit manufacturer’s installation and operating instructions. Usually, no instructions or trap guidelines are included within the unit installation manual.
The information that the contractor requires to properly design and install a P-Trap is: I. Pressure level, negative or positive II. The maximum operating pressure across the trap: drain pan to ambient III. Maximum gallons of condensate per hour to size the drain pipe IV. The greatest interval of time that condensate will not be produced V. The inches per month water evaporation rate for the geographic area VI. Design guide for P-Traps
Most of this information is not available to the trap designer or installer. And even if he had the information, he would not use it. In fact, a great number of contractors do not even install a trap… they just let the air flow in or out of the drain pan area. A contractor recently replaced five RTUs that were not that old but needed replacement because of considerable rust in the cooling section caused by ambient air being sucked in through the drain opening and preventing the condensate from flowing out of the drain into the drain line. The five replaced RTUs were all connected to a central drain line and none were trapped. Rusting, deterioration of thermal insulation, bearing failure, and shorting of fan motor are all caused by the water geyser that results when water is attempting to escape from the drain. The rush of incoming air blows the condensate all over the cooling coil compartment. The new installation, Fig 1, is correctly installed with HVAC Air-Traps placed between the RTU and the main drain line.
Figure 1. Des Champs HVAC Air-Trap Model RLC-C
DRAFT Most installations do not allow for the proper P-Trap design because there is not enough height to have the trap retain the necessary amount of water to prevent dry-out. In fact, the trap will fail to operate properly at some point during the year. If the unit is in a tropical environment the P-Trap will work properly all year if cleaned on a regular basis to remove the sludge and growth buildup at the bottom of the trap. If installed in the arid west it will not seal against air flow for most of the year unless the P-Trap is primed on a regular basis (the trap will dry out at the rate of over 2 inches per month). In other areas of the country, the trap will either freeze and break or with no water will pass air. If heat tape is applied to keep the trap from freezing, then it’s almost guaranteed that the trap will be dry during winter months. As an example, for a trap to not dry out, and pass air, it would have to be designed to hold at least 8 inches of water at the end of a cooling season to seal air during the heating season and operate properly at the beginning of the next cooling season: this would require thermal insulation or heat tracing if the trap is exposed to freezing temperature. Basically, when using a P-Trap to remove condensate from an HVAC unit, you will encounter considerable air leakage to or from the occupied space during a year. The author’s estimate of the percentage of time that a P-Trap operate dry is: Southeast 10%, Northeast 40%, and West 80%.
Air that leaks through a P-Trap has usually just been conditioned to cool or heat the space, so the energy lost is greater than for the same amount of room air leaking to ambient. It doesn’t make any difference whether the conditioned air is being blown out of the unit or drawn into the unit, the wasted air must be replenished, meaning that the makeup air must be filtered, heated, and cooled from ambient to supply conditions.
Rate at Which Water Evaporates within a P-Trap Tests by the author indicate that water will evaporate at a rate of approximately 2.2 inches per month from late spring through early fall. It does not make a difference if the drain pipe is 3/4-inch in diameter or 1 1/2-inch, or from a 20'x40' swimming pool, the water evaporates at an average rate of 2.2 inches in height per month. The readings were taken near Roanoke, VA. If there is a drain line of considerable length that has pockets of water remaining in the line after condensate ceases, then the evaporation rate in a P-Trap could be as low as 1.1 inches per month. In arid regions the evaporation rate will be higher, up to 3 inches per month. For example, if you have a residence in Las Vegas, and are gone for six weeks, the toilet will have dried out in that time period. Basically, dry P-Traps waste considerable energy as will be presented in the following. A typical P-Trap design for draw-through equipment is shown below. For a system operating at 1½ inches of negative pressure, J will be 1¼ inches of height, which with an evaporation rate of 2 inches per month the seal will be lost after just two weeks of noncondensing operation! This is good reason to believe that P-Traps in most of the country run dry over 50% of the time.
DRAFT
Comparison of Air Leakage through a dry P-Trap and a HVAC Air-Trap when Cooling a Medium sized Data Center Leakage Rate of Air through a Dry P-Trap
v = velocity of air within drain line, ft/sec dp = loss of pressure due to flow through the pipes, ounces/in2 d = inside diameter of condensate drain pipe in inches, in L = length of condensate drain pipe in feet, ft N = number of similar traps within conditioned space Qi = volumetric flow per trap, ft³/min Qt = total volumetric flow through N traps within conditioned space 25,000 = unit conversion factor dp = -1.5 inches WC 0.8664 ounces/in² d = 1.50 in L = 5.00 ft N = 200 traps v = (25000 * dp * d / L) ^0.5 ft/sec Air velocity through drain line v = 80.6 ft/sec 4,836.6 ft/min Velocity entering or leaving drain pan Flow area = 0.0123 ft² Drain line flow area Qi = 59.3 ft³/min Airflow to or from cooling unit through drain line Qt = 11,865 ft³/min Total air flow through N number traps: conditioned makeup air is required to replace this escaped air. Leakage Rate of Air through an HVAC Air-Trap™ The specific relationship among the pressure within the drain pan compartment, the ambient pressure, and the flow area between the float valve and the seat is expressed by
Qi = 2610 * A * (ΔP) ^0.5 Where: ΔP = pressure drop across float valve seat, inches of water, which is = P1 - P2 A = open area for flow between the float valve and the seat, ft2 2610 = unit conversion factor dimensionless D = contact diameter between float and seat, inches D = 0.627 in P1 = -1.50 in wc within drain pan P2 = 0.00 in wc ambient Peripheral Gap, ≈ 0.002 in (average distance between float valve and seat) A = 2.7E-05 ft² Qi = 0.087 ft³/min N = 200 Qt = 17 ft³/min Total air flow through all similar traps: conditioned makeup air is required to replace this leakage
DRAFT What the above comparison indicates, on a data center having 200 P-Traps installed in cooling units, is that there is leakage of 11,864 CFM to ambient, whereas the Des Champs HVAC Air-Traps leak only 54.9 CFM total for all 200 traps. The HVAC Air-Trap was initially created and designed for the specific purpose of preventing large amounts of air leakage in data centers. With P-Traps, the operations personnel were having to fill the traps almost on a weekly basis. This was because the air temperature within the cooling coil compartment is a warm 73⁰F and the coils produced condensate for only a few hours per year, like the arid regions of the country. The loss of so much air from the conditioned space, when using P-Traps, caused the data room to go negative. Without changing to Des Champs HVAC Air-Traps they would have had to add 11,000 CFM of fully conditioned make up air with a hefty initial cost and continuous operating cost.
Now, let’s look at a hypothetical energy loss if you were to consider the conditioned air leakage from all the P-Traps installed in the entire United States. I estimate that there is about 200,000,000 of them on residential, commercial, institutional, and industrial structures. Using an average drain pipe diameter of 1 inch and a pressure at the drain of 1.5 inches the air leakage is 21.5 CFM per trap. Assume, conservatively, that the HVAC units have the fans operating 50% of the time and that they are dry 50% during time of operation. Over a year’s time the total leakage of air to ambient would be:
CFMt = 21.5 x 200,000,000 x 0.5 x 0.5 = 1,075,000,000 Average ∆T during year between delivery temperature and ambient temperature is 45F⁰ Btu/hr = 1.08 x CFM x (average ∆T) = 5.22 x 1010 kW = 15,311,494 kW-hrs/year = 15,311,494 x 8760 = 1.34 x 1011 at $0.14 per kW-hr Yearly Cost of replacing leaked air = $18,800,000,000.00
Sealing this leakage should be seriously considered by energy conscious owners and government officials. The quickest, easiest, and most cost-effective way to reduce the wasted energy is to replace the installed traps with Des Champs HVAC Air-Traps and specify them to be used on new installations. Not only would the HVAC Air-Traps be saving “tons” on energy, but you would be eliminating the eight issues inherent with P-Traps:
1) Freezing and breaking 2) Dry-out 3) Sludge formation in bottom of “U” bend 4) Geyer effect of air rushing into drain pan and spraying water or causing overflow of drain pan 5) Mold, mildew, and undesirable bacteria forming in the coiling coil compartment 6) Air leakage 7) Height requirement for trap to function properly 8) Incorrectly designed and installed
Owners would really like saving the energy and not dealing with eight issues resulting from P-Traps, especially water running down their
https://m.youtube.com/watch?v=Cuy2cOGFfl0walls.
