If you're a Air conditioning/R specialist, it is best to check any air-source electric (ASHP) for correct air flow over the coils when troubleshooting or throughout preventive maintenance.
Correct air flow is of crucial importance to the whole process of any ASHP. Area of the warmth transfer rate is dependent upon the air flow over the indoor and outside coils. When the air flow is wrong, then your warmth transfer rates are incorrect and may drastically modify the equipment's performance.
Accurate air flow measurement is important in troubleshooting any electric system. Actually, no refrigeration test applies when the ventilation volume isn't correct!
ASHP producers stick to the tenants from the Ac, Heating, and Refrigeration Institute (AHRI) with a "Test Stand Value" needing the measured air volume rate, when divided through the measured indoor air-side total capacity, mustn't exceed 37.5 SCFM per 1,000 Btu/h [this can be a more 450 cubic ft each minute (CFM) of air flow across an inside coil per 12,000 Btu/h of capacity]. Most producers make use of an acceptable selection of 350 to 450 CFM per 12,000 Btu/h of capacity, and also over 750 CFM per 12,000 Btu/h of capacity across outside coils (most outside fans move roughly 1,000 CFM, as much as 1,500 CFM, per 12,000 Btu/h of capacity). Within the Air conditioning/R industry, 12,000 Btu/h of capacity is known to like a "Ton" of refrigeration. Typically, most producers concentrate on around 400 CFM per "Ton" when rating their equipment.
Before carrying out any air flow determination, always make certain that registers and grilles are open filters and coils are clean, which blowers and fans are running correctly delivering air flow over the indoor coil and also the outside coil.
You will find various techniques which help determine the air flow amount across an inside coil. The indoor coil is usually checked because the air flow must mix this coil to permit the refrigerant either to absorb (ASHP cooling) or reject (ASHP heating) the warmth towards the appropriate "sink." In summer time, the "sink" may be the outdoors, and throughout winter, the "sink" is inside.
Probably the most broadly utilized techniques when checking ASHP air flow continues to be the "temperature rise" method over the auxiliary or emergency (support) heater(s). This process could be carried out no matter outside ambient temperature.
When carrying out this test, you should never forget that typically, most air source warmth pumps operate with similar air flow no matter mode of operation. In heating or cooling, or throughout emergency warmth or defrost, the electric simply provides the "same" air flow per ton over the indoor coil. The only real change can be done throughout the cooling mode, as water condensing around the indoor coil increases resistance slightly, lowering air flow amounts somewhat. Obviously, you may also change blower speeds and CFM amounts for either heating or cooling on some ASHPs.
To check on air flow (CFM) of the ASHP, you need to perform several dimensions and employ some math. The most popular formula for calculating CFM is:
Emergency Warmth Output(Btu/h)
CFM= -----------------------------
Temperature Rise x 1.08
You have to discover the CFM per Ton traveling with the indoor coil throughout heating or cooling modes. To do this, you need to put the system within the emergency warmth mode and put thermometers within the supply air and return air pathways as near to the air handler as you possibly can without struggling with the radiant effect from the heating units.
In the earlier formula, CFM equals the emergency warmth output in Btu/h. Since you'll be checking heating units, you'll be calculating electrical data. When finding Btu/h creation of the heater, you'll appraise the current and also the amperage in the disconnect for that heater(s) and record the. This can require utilization of a voltmeter as well as an ammeter.
Supply current increased by amperage equals wattage. Wattage increased by 3.413 (Btus per Watt) equals Btu/h and you'll then possess the value for that CFM formula numerator.
Within the CFM formula denominator, temperature rise originates from the main difference within the return air and offer air temperature following the heater(s) has/have stable along with a difference has happened. This difference (sometimes known as TR or &DeltaT) will be increased with a constant of just one.08 to obtain the modified temperature difference. Always seriously consider temperature change when locating the improvement in temperature.
This modified temperature difference may be the denominator for that CFM formula.
Once the formula is finished using the necessary inputs, the reply is the entire shipped air flow (CFM) traveling over the heater in emergency warmth mode. After this you simply divide this value through the tonnage from the installed outside coil to obtain the CFM per ton. The cooling air flow (CFM) is going to be close to this value or slightly lower because of elevated pressure drop from water condensing around the indoor coil throughout cooling. The ASHP compressor heating air flow (CFM) per ton is a similar value to that particular found from emergency warmth. The CFM per ton ought to be within the AHRI range to become acceptable.
If inaccurate ventilation (CFM volume) over the indoor coil (evaporator in cooling, condenser in heating) is decided, this case should be remedied just before further research into the refrigeration cycle.
Identifying ASHP indoor coil air flow (CFM) throughout emergency warmth example:
Two (2) Ton Unit
240 Volts at disconnect
20 Amplifiers at disconnect
Return air = 70ºF
Supply air = 88ºF
CFM = (240 x 20 x 3.413) ÷ (1.08 x 18ºF) = 843
843 CFM ÷ 2 Tons = 421.5 CFM per Ton
(This amount is suitable per AHRI)
It's also wise to look into the outside coil for minimal clearance per manufacturer round the unit from bushes, trees, and decks to permit proper warmth rejection within the summer time and absorption during the cold months. You may also determine the air flow over the outside coil using the product specifications in the manufacturer as proven below. Most ASHP outside coils will typically deliver near to this amount when the coil is installed properly and clear.
Phillip A. Rains
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