download Skype
|
Dimensioning and use of AC/heat pump units.
by Hakan Falk, "Energy Saving Now"
The traditional dimensioning of units give a general selection of unit capacity that is 2 to 3 times more than necessary. Reasons for this is the often the goal of more revenue in combination with very simplified calculations. When this is applied to AC versions with fixed revolutions and fixed output, the functionality become degraded because the fixation on air temperatures and that they will stop at set temperature. The reality is that an AC units function is to both lower the temperature and relative humidity and to work properly it is a need for variable output. To keep the relative humidity in comfortable range is very difficult if the unit is fixed revolutions and over dimensioned, because it only have to work very short periods for the temperature demand. The result is cold but humid air and uncomfortable for most humans. The variable output AC-units will continue and dry the air, even when the output is lowered near the set temperature and maintain a higher comfort level.
The human body is more sensitive to high humidity than high air temperature. With low relative humidity the human body have a fantastic capacity to regulate its comfort level. To exemplify this, we only have to think of how different types of saunas are perceived. A typical wet sauna is very hot at 50 degree Celsius and at 80 degree it is fatal for humans, this because the body cannot cool down efficiently with the help of transpiration. A dry Finnish sauna will feel warm at 50 degree, but it is too cold for taking a sauna, it will function at temperatures above 80 degree and normal temperatures are 95 to 110 degree.
The very common Air Condition (AC) units, that we find almost everywhere, comes today in a few basic versions. They are basically two types, wall mount or split type each in following versions,
- AC unit with fixed revolutions compressor for cooling of space.
The basic function is an AC-unit that is a compressor with an alternate current electrical engine and with fixed speed govern by the current. The AC-unit is regulated by thermostat, which will turn on/off the compressor as needed. If the AC unit is over dimensioned, it will work very short periods to maintain the temperature. This means that the AC unit is not working enough time to also maintain dry air and the result is at the end cold and humid air in the room. Correctly or even under dimensioned, the AC unit will maintain dry air but with sometimes higher temperature, which is more comfortable.
- AC unit with fixed revolutions compressor and heat by resistance for cooling/heating of space.
This version is mostly common in the US or places where heat very seldom is needed. The heating option is very inefficient and cause large energy losses.
- AC reversible unit with fixed revolutions compressor for cooling/heating of space.
This is the most common version, when heating is needed. The heating alternative becomes also inefficient for most of them, since the normal 3:1 gain, will go down to 1:around 0 degree Celsius. The reason is that almost all AC units use rotation compressors that loose its efficiency at 0 degree. Some not common AC units, with industrial strength piston compressors, will lose the efficiency at -10 to -15 degree Celsius and work more efficient for heating.
- AC inverter unit with variable revolutions compressor for cooling of space.
This is new technology for AC-units, but existed for quite some time in equipment for computer rooms and laboratories. The principle is to drive the compressor with a direct current engine and this way be able to regulate the speed and the output for the AC unit. In this case the disadvantages of over dimensioning are avoided and the AC unit can maintain both temperature and humidity at varying loads. The savings are substantial and the are often advertised to give up to 30% energy savings, compared with the fixed speed AC units.
- AC reversible inverter unit with variable revolutions compressor for cooling/heating of space.
This is the version of the inverter AC unit, that is reversible to functioning as heat pump also. This is probably the best buy anyone can do, if combination heating/cooling is needed.
The last 20 years I bought and installed around 20 AC units and in the beginning I used to calculate thee need. I found however a very accurate and efficient simplified dimensioning method of my own. When asking the seller of needed capacity, I always bought half the size or slightly less and this have never failed. When the inverter type of AC units with variable output came to the market, I still buy smaller size, but it is not important any longer if it is half the size or less. Because of the function the inverter type should be the preferred choice when buying a new unit and they claim up to 30% energy savings. Even if the old types with fixed revolutions today sells with large discounts, it is quickly lost in higher energy costs.
At the moment the old type of fixed revolutions units are either heavily discounted or sold in developing countries. It is a dead design compared with variable output units and a continuation of manufacturing and selling the old type is contrary to any energy interests.
Hakan Falk
© EnergySavingNow.com
© Copyright energysavingnow.com 2000.
© Copyrights to Software @ this site
|
|
BIOFUELS ASIA
conference
2008 - 27th-28th March 2008,
Thailand
Energy Savings in
UK
Energy Savings in
USA
"Disturbed children"
and
"Grumpy old men".
|
