HVAC FORMULA

Cooling Coil Calculations Actual Air vs. Standard Air CFM

   

The question begins this way: 

"The performance of your unit is not correct."

Why?

"Because when I calculate the coil load from the stated conditions I do not get the capacity shown"

   

Answer:

When calculating the Total Capacity do not use Qt = 4.5 * cfm * (h₁ – h₂)

Because 4.5 is derived for standard air as follows:
ma = cfm * Density * 60 where the density of standard air = .075 lba/ft³

At 100 DB and 78 WB, the W = .015601 lbw/lba

h = ha + Whg = cpa*T + W*(1061 + .444*T) Btu/lba

h₁ =.24*100 + .015601*(1061 + .444*100) = 41.25 Btu/lba

At 57.50 DB and 57.30 WB, the W = .0099622 lbw/lba

h₂ = .24*57.50 + .0099622*(1061 + .444*57.50) = 24.62 Btu/lba

If you use this equation then you will get the following:

Qt = 4.5 * 15000 * (41.25 – 24.62) = 1,122,525 Btu/hr, or a perceived error of 8.3

Coil programs use actual conditions:

At 100 DB and 78 WB, the W = .015601 lbw/lba, Density = .06914 ft³/lba

 h₁ = 41.25 Btu/lba from above

 At 57.50 DB and 57.30 WB, the W = .0099622 lbw/lba

 h₂= 24.62 Btu/lba above

 ma = CFM*Density*60 = 15000 * .06914 * 60 = 62,226 lba/hr

 Qt = ma * (h[1] – h[2]) = 62226*(41.25 – 24.62) = 1,034,818 Btu/hr

 
When calculating the sensible capacity do not use Qs = 1.1 CFM *(T₁-T₂):

 Because the 1.1 is derived from standard air as follows:

 m_{a *} C_pm = SCFM*Density_Std*(C_pa + W*C_pw)

 m_{a *} C_pm = SCFM *.075 * 60 * (.24 + .0093*.444) = 1.1*SCFM

 If you use this equation then you will get the following:

 Qs = 1.1 * 15000 * (100-57.50) = 701,250 Btu/hr                           

 Coil programs use actual conditions:

 At 100 DB and 78 WB, the W = .015601 lbw/lba, Density = .06914 ft³/lba

 At 57.50 DB and 57.30 WB, the W = .0099622 lbw/lba

 ma = CFM*Density*60 = 15000 * .06914 * 60 = 62226 lba/hr

 Cpm = (C_pa + W*C_pw)                  Btu/(lba – F)

 Qs = ma * Cpm * (T[1] – T[2])

 Qs = 622226*(.24+.015601*.444)*(100-57.5) = 652,988 Btu/hr

 The Total Latent Load is computed from:

 Q_L = Q_t – Q_s

 Using standard Air   Q_L = 1,034,818 - 701,250 = 333,568 Btu/hr

 Using actual air:  Q_L = 1,034,818 - 652,988 = 381,830 Btu/hr 

The above shows a sea level calculation.
The same equations apply for altitude however the true density must include DB, WB, and PB = altitude PB. PB altitude is calculated from the following equation:  PB = 14.696 * (1 - ALTITUDE*6.8753E-6)^5.2559

Overhead Electrical Power-Line Distribution

Methods of Feeding Overhead Electrical Power-Line Distribution Lines With BPL Signals and the Relationship of These Methods to the Radiated Emissions of the Conductors

1. Introduction 1.1 There are differences in the way that medium-voltage (MV)2 power-distribution lines conduct and radiate signals based on the way that RF power is fed to the lines. ARRL used a well-known antenna-modeling program, EZNEC PRO3 3.0 with the NEC-4.1 calculation engine4 to model a simple MV power line and two nearby amateur antennas, conservatively located 30 meters from the lines. A pictorial diagram of the model is shown in Figure 1. 1.2 Tables 1 and 2 show the results ARRL obtained by modeling three different ways of feeding the antenna: o Differential feed between two phases, at one end o One phase to earth ground, in the center o One phase fed differentially similar to the way a dipole antenna is fed, offset on the ungrounded phase

2. Description of the Model 2.1 The power-line radiator antenna model was configured with two 12.7 mm copper conductors5, 200 meters in length. They were placed 10 meters above ground. The ground was modeled with average conductivity and dielectric constant. The two conductors were parallel, spaced 1.0 meter. One of the conductors was grounded to simulate typical imbalance in the line. The ground connection consisted of four 10-meter radials, 5 cm above ground. This is a relatively poor RF ground, to simulate the typical poor RF characteristics of power-line grounds. (This also allows those that don’t have access to the NEC-4.1 software to duplicate the results using the more available NEC-2 calculation engine, which cannot handle direct ground connections the same way NEC-4.1 does.) 2.2 Differentially connected loads were placed at each end of the transmission line to properly model the signal losses from various loads present on the line (transformers, BPL modems). This power would not be radiated, so must be accounted for in the model. This also allows the software to calculate the relative efficiency of feeding the system at different points by comparing the power fed to the system and the power that reaches the load, simulating a BPL system modem or repeater. These loads are 50-j0 ohms. 2.3 Two amateur receive antennas are also included in the model. Antenna 1 is a half-wave dipole located 10 meters above ground, at the height of the power line, typical of many amateur tree-mounted antennas. This antenna is 30 meters distant from the line. Antenna 2 is a half-wave dipole located 30 meters above ground, 30 meters diagonally from the line.

The height of this antenna is representative of taller amateur tower installations. Each of these antennas has a 50-j0 ohm load in the center and EZNEC is used to calculate the power that reaches each load by radiation.

Figure 1: This is a pictorial of the model used by ARRL to calculate differences in the performance of BPL systems fed in different ways. Point 1 = Amateur half-wave dipole antenna, 10 meters high, 30 meters from line. Point 2 = Half-wave dipole antenna, 30 meters high, 30 meters diagonally from line. Point 6 = Single-phase differential “dipole” feed point. Points 7 and 8 = Two phase differential feed or load, as specified in Tables. Point 9 = Ground wire, fed where it connects to the phase. Point 10= Earth ground radials (4). 3. Results 3.1 The results of the modeling are shown in Tables 1 and 2.

Sekilas Industrial Process control & instrumentation

titipan dari forum sebelah

Halo temen2 fisika teknik, aq ingin sharing ttg dunia instrumentasi & sistem kendali industri proses (Industrial process control &i instrumentation), merangkum dari pengalaman temen2 alumni yg bekerja di dunia industri proses baik sebagai user, vendor DCS/PLC/SCADA dan kontraktor EPC..

sebagai basic mata kuliah pendukung, kuliah2 basic seperti mekanika fluida, thermodynamic, PPM, kontrol automatis,kontrol proses,dinamika system, sistem pengukuran, pemrograman, sistem digital dll sangat menunjang mempelajari instrumentasi industri

..

industri proses dikatagorikan sebagai industri yg mengolah bahan baku secara kontinyu dalam jumlah besar, seperti oil & gas company, chemical, power plant, fertilizer, petrochemical maupun cement.

sebuah plant proses (exmp: heat exchanger, pressurized vessel dll) dalam pengoperasiannya memerlukan instrumentasi untuk menunjang safety. tingkatan sistem instrumentasi dari yg awal sampai puncak:

1. BPCS (Basic Process Control System)

2. Alarm system

3. Safety Instrumented System (SIS), biasanya Emergency Shutdown System (ESD) & Fire Gas System (FGS)

4. Relief system (pengaplikasian Pressure Relief Valve/PRV)

Sebaiknya dibahas terlebih dulu sistem instrumentasi tingkatan yg pertama,yaitu BPCS.

sebuah loop sistem kendali proses memiliki element sensor, transmitter, controller,algoritma kendali,actuator & final element, serta plant proses/controlled variable (Basic Process Control System/BPCS).

A. Sensor

Variabel proses yg diukur dan dikontrol di industri proses adalah Flow, Pressure, Temp, Level, Analyzer, vibrasi, speed, weigh dll.

contoh:variabel proses yg penting adalah flow, pengukuran flow untuk liquid berbeda dengan fasa vapour,superheated steam & gas, karena liquid bersifat incompressible sehingga pengaruh temperature & pressure actual flowing fluida tidak berpengaruh pada densitas liquid (ingat prinsip thermodynamic).sebaliknya untuk gas, vapour & steam maka diperlukan kompensasi atas perubahan densitas akibat perubahan pressure & temperature flowing.

pengukuran flow ada 2 metode:mass flowrate (ada kompensasi densitas fluida terhadap perubahan Press dan Temp actual) & volume flowrate.

controh:

mass flowrate instrument adalah corriolis,multivariable flowmeter (punya Diff transm,Press transm & temp transm).

Volume flowrate adalah orifice,turbine meter,vortex dll.

kita ambil contoh sistem pengukuran Natural Gas metering (mass flowrate) untuk jual beli, seperti pada beberapa perushan gasl dll:

Intrument yg dipakai:

1.sensor flow yg dipakai adalah orifice (prinsip bernoulli), flow sebanding dengan akar beda tekanan.

2. Differential pressure transmitter, mengukur beda tekanan pada orifice dan menyampaikannya ke recorder/controller.

3.Pressure sensor (tipe diapragm sensor pada pressure transmitter), mengukur kompensasi perubahan densitas gas terhadap pressure.

4.Thermocouple, ngukur suhu actual gas flowing untuk kompensasi perubahan densitas gas terhadap temperatur.

5. Temperature transmitter (biasany electronic 4-20mA)

6. Pressure Control Valve, mengontrol pressure gas yg mengalir ke pembeli.

7. Pressure switch High/High High  (PSH/PSHH) dan pressure switch Low/ Low Low (PSL/PSLL),mendeteksi pressure gas.

   PSH terdeteksi=alarm,PSHH terdeteksi=operasi shutdown, PSL terdeteksi=Alarm, PSLL=operasi Shutdown

8. Shutt Down Valve (SDV), berfungsi sebagai actuator untuk memblock laju aliran gas ke pembeli apabila pressure aliran gas telah mencapai setingan PSLL atau PSHH (INTERLOCK ESD).

9. Gas analyzer,yaitu Gas Chromatograph (GC) untuk mengukur kadar komposisi berat jenis masing2 senyawa kimia dalam Natural Gas seperti metana,butana, Nitrogen dll.

10. Barton chart flow recorder

semua data2 dari sensor/transmitter tersebut dihitung oleh flow computation, dengan menggunakan rumus standard internasionalpengukuran, APPI (klo ga salah) yaitu AGA 3 gas flow calculation (aq ga hafal rumusnya hehe).tp satu hal yg jelas, teori2 thermodynamic & mekanika fluida berperan besar dalam perhitungan AGA 3.   

B.Transmitter

berfungsi mengirimkan sinyal elektric (4-20 mA) atau pneumatic (3-15 Psig) ke controller,proportional dengan nilai ouput sensor.

jenisnya: Pressure Transmitter, Differential Pressure Transm, Temp transm dll

C.Controller

alat pengendali bisa berupa electric/pneumatic controller yg tidak terdistribusi, maupun yg terdistribusi seperti DCS,PLC,SCADA (tuk remote/jarak jauh area)..saat ini PLC sudah memiliki fitur menyamai DCS (pny Human Machine Interface,analog I/O ribuan,PID control,data history & redundant controller),namun ada hal2 prinsip DCS yg tidak dimiliki oleh PLC yaitu (referensi dari vendor ABB):

1. Support Asset Management features

2. Support Safety System, ada istilah Safety Integrity level (SIL),panjang penjelasannya jadi silakan cari di internet saja hehe.

3. Integrated Development Software

untuk Plant yg menuntut safety tinggi, maka DCS lebih unggul daripada PLC, dgn adanya SIL 3 untuk DCS.untuk Pembangkit Listrik

Tenaga Nuklir (PLTN) maka memiliki SIL paling tinggi yaitu SIL 4.

D. Algoritma kendali

saat ini algoritma PID (Proportional Integral Derivative) masih umum digunakan, setahuku ada Model Predictive Control (MPC) yg mulai tersedia di salah satu vendor DCS. penggunaan MPC menandai babak baru sistem kendali proses, yaitu Advanced Process Control (APC) untuk optimasi sistem kendali.

PID control adalah Basic Regulatory Control System pada sebuah feedback control,namun di industri,sebuah proses plant memiliki banyak variable proses sehingga muncul istilah cascade, ratio, feedforward,three element,decoupler control (Advanced Regulatory Control) yg tetap menggunakan PID control sebagai basic algoritma.

E. Actuator & final element

biasanya adalah control valve, damper, motor (untuk speed & berat pada conveyor). yg populer di industri proses adalah control valve.

untuk regulatory (pengaturan) control:Pressure Control Valve (PCV), Flow Control Valve (FCV), Temp Control Valve (TCv), Level Control Valve (LCV), Hand Control Valve (HCV).

untuk On-Off control ; Motor Operated Valve (MOV), Remote Operated Valve (ROV).

untuk Emergency Shutt Down system (ESD): Shutt Down Valve (SDV),Blow Down valve (BDV).

penggunaan valve untuk regualtory, On-Off dan ESD masing2 memiliki karakteristik yg berbeda.

F. Plant proses

plant proses adalah tempat dimana berlangsungnya suatu proses, contoh sebuah tangki yg didalamny bisa terdapat beberapa feedback control seperti Level Control System, Pressure Control System maupun Flow Control System dan terdapat cascade, ratio,decoupler control.penguasaan terhadap proses2 fisika pada suatu plant, dari pengalaman,seharusnya intrument & process control engineer harus bisa sehingga bisa menjabarkan dinamika system pada suatu plant. ada hubungannya nanti dengan proses penentuan nilai variabel2 Tuning PID (Proportional band, reset time(I) dan derivative time) dari orde proses yg dihadapi maupun penentuan jenis regulatory apa yg cocok dgn proses entah itu cascade, ratio, feedforward dll.  

Speed Control Motor untuk PLC

Contoh AC Motor Speed Control System:
Contoh Konfigurasi Sistem dari AC Motor Speed Control System:
Contoh Panel kontrol dari AC Speed Control Motor
The speed can be set easily with the potentiometer on the speed controller front panel, And Connecting to PLC with Rear Panel. Kecepatan dapat diatur dengan mudah dengan potensiometer pada kecepatan controller panel depan, Dan Menghubungkan ke PLC dengan Rear Panel.
1. Front Panel :  
2. Rear Panel :  
Contoh Menghubungkan AC Motor Speed Control System untuk Input dan Output PLC:
Connection Diagram with PLC Diagram koneksi dengan PLC
This connection diagram shows an example of three-phase 200-240 VAC Specifications. Diagram hubungan ini menunjukkan contoh dari tiga fase Spesifikasi 200-240 VAC.