Two-Hose Units in Residential Applications
July 9, 2007
Portable air conditioning in the residential market is a growing industry segment. The need filled by these products is supplementary air conditioning for a space such as a bedroom, room addition, or other area where additional air conditioning might be required but not available from the residence’s air conditioning system.
There are various ways to provide portable air conditioning, and in the residential market the predominant designs are one-pipe or two-pipe systems.
Which to use?
Answer: It depends.
The single-hose system uses room air to cool the condenser. The heated condenser discharged air is then exhausted to the outdoors through the single hose. This design has the advantage of reduced condenser air restriction, with one hose instead of two, which may provide more air through the condenser. This increased air will reduce the condensing temperature - all things being equal - and such a temperature reduction will increase the unit’s overall capacity.
Two-hose systems operate in a similar manner to the single-hose system, except the inlet to the condenser is not from the conditioned space, but is from the outdoors. The heated condenser air, as with the single-hose system, is discharged to the outdoors.
BREAKDOWN OF SYSTEMSPer ASHRAE Standard 128, Method of Rating Unitary Spot Air Conditioners, these two designs are rated at the same conditions for both the evaporator and the condenser: 95°F dry bulb, 83° wet bulb. In the case of rating per the standard, additional condenser air will be an advantage to the unit’s capacity, so a single-hose system appears to be the winner.
However, there is more to consider. As a supplementary cooling system, there is a vast difference between the actual overall performances of the two types of units.
In reality, the evaporator in both units will see fairly normal temperatures for a residence; say 80° dry bulb and 50 percent relative humidity (67° wet bulb). The single-hose system also uses this relatively cool air as condenser entering air, and lower inlet air temperature will be reflected in better overall capacity. While this all sounds good, there is a major problem: Air exhausted from a home must be made up from somewhere.
If the home is kept at the normal 80° dry bulb and 67° wet bulb, and the outdoor air is the normal rated 95° dry bulb and 75° wet bulb, the amount of heat brought into the home to make up for the exhausted condenser air on a single-hose unit can be significant. A typical 1-ton air conditioner will exhaust approximately 600 cfm as condenser air.
The amount of heat brought into the home to make up this air under these conditions is shown in Figure 1.
If the condenser air were not 600 cfm, but actually 400 cfm, the heat gain would still be more than the cooling capacity, or 12,366 Btuh.
In this example, a single-hose system could be considered a losing proposition. A 1-ton, 12,000-Btuh single-hose unit requires over 18,000 Btuh to make up for the heat brought in by the condenser. In short, the single-hose units are not meant for this type of overall cooling application. They provide a blast of cool air to whoever is standing by the discharge, but when applied to a home, the net effect on that home is a substantial loss of cooling that must be made up by the central air conditioner, at a cost. If there is no other air conditioning, this could make the home even warmer.
A two-hose system, on the other hand, does not have this inherent problem. No air must be made up. All condenser air is brought in from the outdoors through the condenser inlet hose.
Admittedly, there is a loss of capacity due to the warmer condenser inlet air. This capacity loss may be 10-15 percent, but is nowhere near the loss of the single-hose system.
For residential, closed system applications, if the portable unit is only needed for spot cooling, a one-hose system could be used. However, the two-hose condenser system should be the product of choice when trying to cool an area in a home.
Publication date: 07/09/2007