Home > Residential HVAC > It Ain't Pretty When Condensing Units are Ugly It Ain't Pretty When Condensing Units are Ugly I've followed other contractors onto several jobs this year where the system had poor cooling, high electric bills, and were running a higher-than-expected head pressure. One thing all these systems had in common was that the “ugly” condenser unit was hidden inside an enclosure. It seems that many newer housing developments have it in their bylaws that the heat pump or air conditioning outdoor unit must be hidden from view. So the general contractor builds a wall all the way around it. Sure, except for the simple fact that outdoor air can't get inside the enclosure, so the discharge air from the condensing unit re-circulates back through the condenser coils again. There are other things that can cause condenser airflow to re-circulate. Placing a condenser under a building or deck is common practice at condominiums in my area. Dense bushes can have the same effect as a fence enclosure.

I went on a call recently when it was 95F outside. Inside the enclosure, next to the condenser, it was 115F. The system was running constantly, but it just couldn't keep up with the cooling load placed on it. A check of the manufacturer's data shows what happened. The system was rated at 3 tons. At AHRI conditions (80F dry bulb, 67F wet bulb indoors and 95F dry bulb outdoors), the cooling capacity was 35,600 BTU. However, at 115F outdoors, the cooling capacity was only 31,200 BTU. That's a 12% capacity loss of 4,400 BTU, or more than one-third of a ton. Think of what that does to the electric bill, and how it affects comfort. So what's the solution? The best thing would be to leave the outdoor unit out in the open. In areas that don't allow that, consider using a lattice enclosure with a high percentage of opening. Or, just block the side facing the street so that at least two sides are open. Other options are to leave a 6-in. gap at the bottom of he enclosure so air can get in all around, or make the enclosure significantly bigger than the outdoor unit.

Anything that restricts or recirculates airflow at the condenser reduces the performance of the system. Remember, air conditioning begins with air. Kevin O'Neill, CM, is the co-owner of O'Neill-Bagwell Cooling & Heating, Myrtle Beach, S.C. He has 31 years experience in the HVAC service business, is a 24-year member of RSES, and was a finalist in the 2005 NATE Certified Technician Competition. Kevin can be reached at 843/385-2220; I. Lattices and Unit Cells So far we have only considered rotational symmetry. This next section takes a look at the translational symmetry which must be present for a crystal to diffract an X-ray beam and produce Consider a thin section through a small-molecule crystal structure Although the slice is essentially two-dimensional, most of the following arguments apply equally well to a three dimensional system. The purple-coloured atoms can be joined by straight lines as shown to form a grid or two-dimensional lattice, the basic repeat unit of which is a parallelogram.

In order to describe our crystal structure, all we need is a description of the contents of one of these parallelograms plus the symmetry of lattice translation. By repeating the contents of the parallelogram ad infinitum,home inspection ac unit all of space canlife of a air conditioning unitA similar situtation may be observed with commercial wallpaper.charging a home air conditioning unit The lattice itself is actually an infinite set of equally-spaced points: It is not the lines which connect the points. (These lines simply provide a useful guide to the eye.) In the real space of a crystal, the origin of the lattice is an arbitrary point chosen by the crystallographer. Thus, an identical lattice would be obtained if, in the above figure,

the positions of the dark-green coloured atoms had been chosen as lattice points. (Note the contrast to the reciprocal-space lattice used to identify diffraction spots which has a natural origin corresponding to the straight through X-ray beam.) In three-dimensional space, the equivalent repeat unit is a parallelepiped known as the unit cell as shown below: The translations in a 3-dimensional lattice may be described in terms of three linearly independent, i.e. non coplanar, vectors, The angles between the pairs of vectors and , and , and and are defined as , , and , respectively. The faces of the unit cell containing the pairs of vector and , and , and and are referred to as the A, B, and C faces, respectively. Considering again the "2-dimensional" crystal structure shown above, then the lattice points may be drawn as small purple dots as shown below, based on the purple balls in the figure above: The question arises as to the choice of parallelogram for our two-dimensional

The black lattice lines are a result of choosing the unit cellHowever, one could equally choose the unit cells shown in red,The unit cell shown in cyan is clearly identical to the green one, but has a different choice of origin. an origin that is convenient to describe the contents of the unit cell. Frequently the origin corresponds to a point of high symmetry within the unit cell: for example, it is often sited on an axis or where two or more axes intersect. Crystallographers also choose unit cells which contain the smallest numberThe unit cell shown in blue contains two lattice points (one at the origin corner and one in the middle) whereas the others allThe blue cell is referred to as a centred unit cell; containing only one lattice point are all described as primitive. A primitive unit cell can always be chosen for any two- or crystallographers choose centred cells when the centred cell displays the symmetry of the lattice better, e.g. the unit cell may have certain

angles constrained to be 90° indicating the presence of an axis (see next page). Both the red and green unit cells shown above are the same area and haveWhich is the best choice of unit cell? The red cell has a larger internal angle than the green cell. When the cell angle is not constrained by symmetry, crystallographers usually choose a unit cell such that the cell angles are as close to 90° as possible, and in the above example the green unit cell is the preferred choice. The table below lists the various possibilities for centred unit cells. The multiplicity is the ratio of the volume of the centred unit cell to that of the equivalent primitive one. It is important to realise that there are two real-space coordinate systems used by crystallographers: The position of atoms, r, within the unit cell may be given by the vector expression: r = xa + yb + zc These 3-dimensional coordinates, x,y,z, are called fractional coordinates, which range from 0 to 1.