Valve Selection Procedure - Carel E2V Manual De Instrucciones

1. INTRODUCTION
The expansion capacity of a valve is determined by the pressure difference ΔPV immediately upstream and downstream of the valve.
The size of the valve must therefore be chosen based on the maximum fl ow-rate and the operating status in which the pressure head ΔPV at the ports is at the
lowest value, and consequently with the minimum pressure Pin of the refrigerant at the inlet and simultaneously the maximum pressure Pout at the outlet.
Δ
It should be noted that the pressure difference ΔPV (= Pin – Pout) across the valve is often signifi cantly different from the pressure head
ΔPC (= Pcond – Pevap) generated by the compressor; this is due to:
the pressure drop ΔPH in the valves, the lines, the condenser and the dewatering fi lter between the compressor and the valve;
•
the pressure drop ΔPL in the equaliser, the evaporator, the lines, the valves, the liquid separator (if fi tted);
•
• the pressure column due to the water column of the pipes between the condenser and the valve, which is equal to the product of the difference in height
DH by the density of the liquid, and is approximately equal to 0.1 bar per metre.
In addition, the liquid inlet temperature has a signifi cant infl uence on the cooling capacity of the valve.
In fact, for the same mass fl ow-rate of expanded refrigerant and operating pressure, the cooling capacity delivered increases considerably as the temperature of
the liquid Tliq decreases (this must in any case be lower than the Saturated condensing temperature Tcond, due to subcooling, so as to prevent the valve from
taking in vapour and causing a decline in performance).
2. DESIGN DATA
To size the valve using the Selection sheet, the following design data must be available:
a. Type of refrigerant used
b. Tcond, Tevap (°C) = Design saturated condensing and evaporating temperature (corresponding to Pcond, Pevap)
c. CAP (kW) = Cooling capacity of the unit in normal operating conditions
ΔPH, ΔPL
d. (bar) = Pressure drop at design conditions in the high and low pressure branches respectively
ΔH
e. (m) = Difference in height between the condenser and the expansion valve
f. Tliq (°C) = Temperature of the liquid refrigerant at the valve inlet

3. VALVE SELECTION PROCEDURE

Establish the design pressure head ΔPC (= Pcond – Pevap) in bars;
1.
The minimum outlet pressure Pcond and the maximum suction pressure Pevap available should be used.
If, rather than the pressure, the saturated condensing temperature Tcond and saturated evaporating temperature Tevap are known, calculate ΔPC from Table
1 in the Selection sheet relating to the chosen refrigerating.
Calculate the pressure difference ΔPV across the valve by subtracting from the pressure drop ΔPH and ΔPL in the high and low pressure branches from the
2.
pressure head ΔPC (= Pcond – Pevap), and taking account of the pressure column, according to the following formula (ΔH is expressed in metres):
N.B.: the factor 0.1 × ΔH (to be neglected if ΔH < 3-4 m) must be added sif the condenser is higher than the valve and vice-versa subtracted.
3. Determine the temperature of the liquid Tliq at the valve inlet and in Table 2 identify the Correction Factor CF to keep account of the cooling capacity of the
refrigerant. If more precise information is not available, assume Tliq = Tcond – 5°C.
4. Multiply the cooling capacity CAP by the coeffi cient CF getting the capacity RATING equal value of the valve.
In Table 3 identify the cell relating to the pressure difference that is closest to the ΔPV calculated in point 2. Based on the saturated evaporating temperature Tevap
5.
determine the model of valve whose capacity is immediately higher than the RATING value calculated above.
lr
Tliq
ΔPV = ΔPC - ΔPH - ΔPL + 0,1 × ΔH
5
EXV Expansion valve
ev Evaporator\\
sa Liquid accumulator
k Compressor
co Condenser
lr Liquid receiver
sv Solenoid valve
fd+sg Dewatering fi lter + fl ow indicator
Δ
Pcond Compressor discharge pressure
Δ
H
Tcond Saturated discharge temperature
Pevap Compressor suction pressure
Tevap Saturated suction temperature
Pin Valve inlet pressure
Pout Valve outlet pressure
Tliq Effective liquid inlet temperature
ΔPC
Pressure head (Pcond – Pevap)
ΔPV
Pressure difference across the valve
ΔPL
Pressure drop in the low pressure branch
ΔPH
Pressure drop in the high pressure branch
ΔH
Condenser/valve height difference
+030220815 rel. 1.0 del 08.05.07