Wednesday, January 14, 2015

3 RHVAC REPAIR (body analogy): Superheat, Subcooling, Delta T


SUPERHEAT - WHAT'S GOING ON IN EVAPORATOR

High Superheat = Thirsty Evaporator (little refrigerant)
Low Superheat  = Flooded Evaporator

SUBCOOLING - WHAT'S GOING ON IN CONDENSER

High Subcooling = Flooded Condenser
Low Subcooling  = Starved Condenser (little refrigerant)


Body Analogy:
Body is Hot = Thirsty (high superheat)
Body is Cool = Drank plenty of water (high subcooling)



SUPERHEAT & SUBCOOLING

High Superheat & High Sub-cooling:

 -- restriction/blockage in coil, orifice or line set
 -- too little refrigerant in low side (suction line)
 -- too much liquid refrigerant in the high side (liquid line)
 -- Evaporator starved of refrigerant
 -- restricted TXV or drier
 -- check TXV bulb tightness
 -- check insulation)


Low Superheat & Low Sub-cooling:

 -- orifice too big
 -- no orifice in the unit/orifice is stuck and refrigerant is by-passing it
 -- Evaporator flooded with refrigerant
 -- TXV opened too much


High Superheat & Low Sub-cooling:

 -- Undercharged on both sides (suction line & liquid line)
 -- find the leak


Low Superheat & High Sub-cooling:

 -- Overcharged on both sides (suction line & liquid line)
 -- remove, adjust charge




TROUBLESHOOTING: SUPERHEAT, SUBCOOLING, DELTA T


1. LOW CHARGE

 --- High superheat
 --- Low subcooling
 --- Low indoor TD
 --- Low suction pressure
 --- Low head pressure
 --- Low compressor amp draw


2. OVER CHARGE

 --- Low superheat
 --- Normal indoor TD
 --- High subcooling
 --- High suction pressure
 --- High head pressure
 --- High compressor amp draw


3. LOW INDOOR AIR FLOW
 --- Low superheat
 --- Low suction pressure
 --- Low to normal head pressure
 --- High to normal subcooling
 --- High indoor TD
 --- Minimal effect on current draw
 --- Low evaporator air flow
 --- dirty filters
 --- dirty evaporator coil
 --- Evaporator coil may freeze up


4. LOW OUTDOOR AIR FLOW

 --- Low subcooling
 --- Low indoor TD
 --- High superheat
 --- High suction pressure
 --- High head pressure
 --- High outdoor TD
 --- High current draw
 --- dirty condenser coil
 --- bad condenser fan


5. RESTRICTION

 --- High superheat
 --- High subcooling
 --- High head pressure
 --- High amp draw
 --- Low suction pressure
 --- Low indoor TD


6. WEAK COMPRESSOR VALVES

 --- Low superheat
 --- Low head pressure
 --- Low indoor TD
 --- Low current draw
 --- High subcooling
 --- High suction pressure




CRITICAL TEMPERATURE DIFFERENTIALS (depends on refrigerant)
:

Evaporator Delta T --- NOT to exceed 20 F

Condenser Delta T --- NOT to exceed 30 F

Evaporator Superheat --- between 20 F and 30 F

Condenser Subcooling --- NOT to exceed 15 F



Note:
Delta T conversion (degrees F to C)
Subtract first and then convert

F = 1.8 * C

C = F/1.8


Example:
T1 = 75 F
T2 = 90 F

Subtract:
Delta T = T2 - T1
Delta T in F = 90 - 75
Delta T in F = 15 F

Convert Delta T in Degrees F to Degrees C:

C = F/1.8
Delta T in C = 15/1.8
Delta T in C = 8 C




TYPICAL VALUES (depending on refrigerant):


SUPERHEAT: 10F - 15F --- short suction line lengths (less than 30 ft.)
SUPERHEAT: 15F - 20F --- longer lengths (between 30 and 50 ft.)

SUPERHEAT: 12-15 degrees F --- when ambient outside air temp is 75-85 degrees F
SUPERHEAT: 8-12 degrees F --- if the ambient temperature is 85 degrees F or over

Superheat with TXV: Nominal 10 degrees F at evaporator outlet
Superheat with piston: 8 to 20 degrees F at suction service valve

Superheat with TXV/cap tube: 8°F to 20°F
Superheat with electronic expansion valves & solid state controllers (newer systems): 5°F to 10°F

Subcooling: 8 to 12 degrees F
Sub-Cooling: 12-15 degrees F

EVAPORATOR DELTA T: 15F - 20F --- airside delta T across the evaporator 
EVAPORATOR DELTA T: 15-18 degrees F

AIRFLOW ACROSS EVAPORATOR: 350 to 400 cfm per ton of cooling capacity

Rule-of-thumb charging:
 -- Units come charged with refrigerant for 15 ft lineset
 -- Add 0.6 oz refrigerant per foot over 15 ft




UNIT COSTING:

$500 - $700 per ton capacity

example: 2.5 Ton Air-Conditioner (18 SEER) costs $2000
example: 120 Tons costs $60,000




INSTALLATION & LABOR COST Estimate:

Unit cost X 2

Heat Pump example:
4 tons unit for 4 bedroom (2500-3000 sq ft) house
4 tons unit cost: $3000 (18 SEER, 10 HSPF) heat pump
Installation Labor cost: $6500




APPROXIMATE TONNAGE SIZING:

500 sq ft per ton
50  sq m per ton

Area conversion:
10 sq ft = 1 sq m




Capacity conversions:
1 Ton = 12,000 Btu/hr
1 Ton = 3.5 kW
1 Ton = 4.7 HP



See also:
1. Basics of Refrigeration and Air Conditioning
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#RX#

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