BF,, ADP and SHF Relationsh BF R elationship ip Worked Examples June 29, 2013
The following determinations have been made with regard to a space to be air conditioned.
Room sensible-heat gain 120,000 Btu/hr Room latent-heat gain 35,000 Btu/hr Room design conditions 76°F DB, 50 percent RH Assume 100 percent percent recirculated air and a bypass factor factor of zero, and determine: determine: 1. 2.
The room apparatus dew point The quantity of air to be circulated
Solution
Draw a line on your psychrometric chart connecting SHF = 0.775 to the reference point at 80°F DB, 50 percent RH. Parallel to this line draw another line through the room design state point (76°F DB, 50 percent RH) to intersect the saturation line. 1. Read ADP = 51°F (Room ADP = Coil ADP when BPF = 0)
2. In the following example, the problem of bypass factor will be included, still with 100 percent recirculated air.
F IG . 1 0 G r a p h i c a l r e p r e s e n t at i o n o f t h e r e l a ti o n s h i p b e t w e e n a p p a r a t u s d e w p o i n t ( A D P) , s e n s i b l e h e a t f a c t o r (SHF), air-off-coil temp erature, and by pass factor (BPF), with 100 percent recirc ulated air assum ed.
The ADP is located as above (see Fig. 10), but the leaving-air temperature (point B) is now greater than the ADP temperature, since some of the air has not contacted the cold coil surface. The temperature of the leaving air can be calculated from the relation where tla = tr - (1 – BPF)( tr - ADP) t la=dry-bulb
temperature of air leaving the coil of room air (entering coil) when 100 percent recirculation is assumed t r =temperature The difference in temperature which is available for heat pickup is then
Example For the conditions given in previous example reiterated but with a coil bypass factor of 0.2,
Room sensible-heat gain120,000 Btu/hr Room latent-heat gain 35,000 Btu/hr
Room design conditions 76°F DB, 50 percent RH find 1. 2.
The temperature of the leaving air The quantity of air required
Solution As before, SHF = 0.775 and ADP = 51°F. o
1. t la = 76 – (1 – 0.2) (76 – 51) = 56 F
2. Note that, as coil efficiency drops, the quantity of air required to remove room sensible heat and room latent heat in the proper proportions increases markedly. As athird example. the effect of introducing outside air will be discussed. Referring to Fig. 11 below, let 0 represent the state point for the outside air and A the state point for the ro om air. Point C, the condition of the mixed air entering the coil, is determined by the method previously described. AR is the room sensible heat condition line and CX is the entering air sensible heat condition line or coil process line. R is the room apparatus dew point, and X is the coil apparatus dew point.
F IG . 1 1 G r a p h i c a l r e p r e s e n t at i o n o f t h e r e l a ti o n s h i p b e t w e e n a p p a r a t u s d e w p o i n t ( A D P ), s e n s i b l e h e a t f a c t o r ( S HF ), a n d b y p a s s f a c t o r ( B P F ) w i t h a m i x t u r e o f r e t u r n a i r a n d o u t s i d e a i r .
Careful analysis of the diagram reveals that:
1.
The cooling coil will have to be operated at a lower temperature (coil apparatus dew point) when warm outside air is introduced, in order to maintain the room apparatus dew point required by the SHF and the room design conditions. This requirement increases the refrigeration load. 2. No change in the total air quantity is required, since room conditions are the same and therefore air-off-the-coil conditions must be the same as when 100 percent recirculation is used. 3. The outside air introduced increases the refrigeration (or total) load but does not increase the room load appreciably unless the coil bypass factor is high. Example A suite of offices is to be air conditioned. The following determinations have been made:
Outside design 95°F DB, 82°F WB Inside design 78°F DB, 50 percent RH Room sensible-heat gain 150,000 Btu/hr Room latent-heat gain 30,000 Btu/hr Ventilation (outside) air through conditioner2000 cfm A four-row DX coil (BPF = 0.10) will be used. Determine the following: 1. 2. 3. 4. 5. 6. 7. 8.
The room ADP temperature The temperature of the air leaving the coil, t la The total quantity of air required (in cubic feet per minute) The temperature of the air entering the coil (mixed outside and return air) The coil ADP temperature Coil operating temperature Total refrigeration capacity Effect of coil bypass
Solution 1. Room ADP
Draw the reference line and then a line ( AR of Fig. 11) parallel to it through the room condition point to the saturation line, and read room ADP = 55°F. 2. Leaving air temperature. o
tla = 78 – (1 – 0.10) (78 – 55) = 57.4 F 3. Quantity of air required
4. Temperature of mixed air entering coil From psychrometric chart
5. Coil ADP On the line connecting the room state point with the outside-air state point (A0 of Fig. 11) plot the point of intersection with the 80.7°F DB line. This is point C of the diagram. From this point draw a line through the point L and on until it intersects the saturation line at point X. Read the coil ADP at 51°F. 6. The significance of coil ADP Its definition of mean coil surface temperature. To produce this average surface temperature, the refrigerant temperarure must be lower than the ADP. Usually 5 to 7 °F is sufficient for efficient heat transfer. o
trefrigerant = ADP – (5 to 7 F) o o o trefrigerant = 51 F – 7 F = 44 F saturated refrigerant temperature for DX coil or average chilled-water temperature for a chilled-water coil 7. Total capacity Total capacity of the refrigeration equipment is the total space load plus outdoor air load.
where subscripts are:
S = sensible L = latent O = outdoors T = total 8. Bypass air The effect of air bypassing through the coil may be analyzed in the following manner:
In effect this becomes additional space load as the by passed air is neither cooled nor dehumidified. Although this would appear to b e a substantial additional space load, it must be kept in mind that the equipment is selected to meet the total capacity so it will balance at a condition sufficiently close to design conditions to be acceptable. Analysis procedures to include partially the effects of coil bypass may be found in some references. Such methods result in somewhat different supply-air conditions but they still specify a point on the room SHF line. A trial and error procedure could be applied which would correct the space SHF line to include coil bypass. The principal difficulty still remains in selecting a coil that would exactly match the calculated specifications. Category: Uncategorized
